Beginners Guide to Computer Images

The aim of this page is to enable you to produce computer images suitable for sending by email or for use on the web. This goal is achieved by reducing file size to a minimum but maintaining picture quality.


You will need some graphics software to manipulate the images. The basic programs included with Windows are unlikely to provide all the necessary functions. Start with the software that comes with your scanner or digital camera. There is some shareware available on the web and trial versions of software for purchase. Try out some of these to find out what you need the software to do for you, before making a purchase.

File size

Bear in mind that an email recipient may have a limited mail box size, or may be paying phone or internet charges by the minute.
When loading files on the web, to a web page or a picture sharing service, remember that some viewers will have slower modems and are unlikely to wait for a large picture to load. Large images will freeze some browsers completely.
Also if an image file is too large for the amount of free RAM on the viewer's computer, attempting to view the image may crash Windows. As a rule of thumb, keep individual web images around 30-50K and the total on any one page under 200K. If that size is insufficiently detailed, then have a link to a separate page containing a larger version of the picture, but tell the viewer that the link leads to a large picture.
Before using a file as an email attachment, or posting it to the web, always look at the size of the file in Windows Explorer.


Picture dimensions and file sizes are related. The bigger the screen dimensions, the bigger the file. Fix the dimensions first.
Ordinary photographs can be enlarged or reduced from negatives. Computer images are not so flexible. Trying to enlarge a computer image will always produce a fuzzy unsharp result. If your image is not large enough, then rescan or take a photograph closer to the object. Computer images can be successfully reduced.


Scan your image at a resolution which produces the size of image you require. Using a higher resolution does not make a sharper picture, if you then have to reduce the image size to fit on the screen. There is no magic number for scanning. The idea that scans should always use 72 dpi is a myth; for an explanation see Scantips.

Saving Files

Save the image file as soon as you have got it on to your PC by scanning, uploading from your camera or copying from a CD. Then take a copy of the file and work on the copy. Every time you complete a step satisfactorily save the file, and take a new copy. Then when anything goes wrong, you can always go back just one step and try again. All the intermediate files can be deleted from your computer when you have finshed.


Remove all background that isn't relevant. Uploads from digital cameras and the files obtained on CD from photo processors often have a lot more background than is needed. For instance, if you take a photograph of people seated at a table, the top half may be the upper part of the room above the heads, so crop the picture to remove it. Scanned items don't usually have extra background.
To remove background, find the selection tool in your graphics software. The selection tool is called a marquee tool in some programs. The icon for it is usually a square drawn in dotted lines. Use the selection tool to select the interesting part of the picture. Then you can copy this part of the picture and paste it as a new image, or crop the existing image to the selection.


Many programs automatically reduce the size of the viewed image to fit on the screen. When images are uploaded to the web, or opened by email recipients in programs that do not scale pictures, the visible image will be far too large to display without scrolling. So view your image at actual size tofind out how large the image really is. You can do this in a simple program that displays images at actual pixel size, such as a web browser. Or you can change the view in a graphics program to actual size. Look for menu items named View or Zoom, then select 1:1, Actual Pixels, Normal Size, Normal Viewing or 100%. Some programs have a drop down box with percentages on the toolbar. Others have a scaling tool for which the usual icon is a magnifying glass (sometimes with a plus or minus in the glass).


Now look at the size in pixels of the picture you have. Will it show on most people's computer screens without scrolling up and down? These days 1280 pixels wide by 1024 high is a medium resolution. Only a small proportion of displays use higher resolutions, because high resolution needs good eyesight and a bigger more expensive monitor. So the aim is to produce a picture file which will display on a screen at 1280 by 1024 resolution so that most users can see the entire picture at one time, and do not need to scroll up and down or side to side.
Files to be sent by email should be no more than 1100 pixels wide by 900 high to allow space around the picture for the program which is displaying them. Files for websites should cater for older and smaller monitors which run at 800 by 600 resolution, so keep web images below 700 by 500.
Look for a tool called Image size, stretch or resize and use this to change the size of the image.

File type

All files to be put on web pages must be saved as either a gif or jpg since these are the file types everyone can view those in their web browser. Any other type of file cannot be seen by all viewers. For a coloured image with many tones jpg's are best, but for line drawings or limited colour sets gifs will render the single colour areas better with less noise (speckles of other colours).
Choosing the appropriate file type for your picture will keep the file size to a minimum. When sending files by email, other formats can be used if you know that the recipient can open them. Some formats tend to produce very large file sizes, for example .bmp and .tiff, as they are intended for other purposes.

Compressing jpgs

The format of a jpg file is 'lossy' - a bit of computer jargon which means that by taking out some information the program displaying the file will make its own judgement what to show in between the remaining data. Taking the information out is called compressing. Even a small amount of compression will reduce the file size a lot. Applying 50% compression to an image can reduce its file size to 20% of the original. Try saving copies at various compressions - it takes a lot of compression before there is any visible effect. Many of the images on this web site have been compressed by 90% or more.
The golden rule of compression is only compress once. Compressing an already compressed file will reduce the picture quality enormously. In compression 30% + 40% does not equal 70%. Take a new copy of the original file every time and compress it. Compression is available in graphics programs like Paint Shop Pro or Adobe Photoshop but not in the simplest ones like Paint. There are some freebie compression programs available on the web.

Sending images in email

How to send an image in email depends on your email software. Look for functions named add attachment, insert file, insert object in help. Type a few words of text in the email as in a very few cases an email with no text and an attachment cannot be opened.

Further information

This page provides only a very brief introduction to computer images. There are lots of websites with more information. One I use is How to scan. A lot of the material on this site is useful for images obtained in other ways, as it deals with how to display the images and how to print them.
In particular there is a section about removing moiré interference when scanning images in books, newspaper and magazines. These same techniques will improve scans of anything with a woven texture, such as fabric, needlework and bobbin lace.

All designs and photographs on this site are copyright Stephanie Peters, except where stated. The designs by Stephanie Peters may be reproduced for personal use but not sold, nor used in any publication without permission. Permission to publish in newsletters etc. will be willingly given to any not for profit organisation that cares to ask.