General Overview of Computer Graphics Technical Process

One of the first questions to ask is:
Are you making graphics for a printed page
OR
Are you making graphics for the computer screen and the web ?

If for the web (computer screen) then you want your images small in size. This does not mean that you need pictures that are miniture. It means that the file size needs to be small. You see the trick is to have the picture look big (high quality) but have the file size be small.

If for printing then you want the file size as large as possible. Small files tend to look jagged when printed. Try it print out a web page the photos will not look very good. But more on this later.

Making images for the Web...
Why small? Ever had to wait for pages to load?
The size of an image (its size on a computer hard drive) is measured in KB. Even at modern Broadband speeds a large than needed image will slow your browsing experience.

So the best way to make an image file size small is to crop it and to resize it. The size of a picture on the screen is measured in pixels (also known as dots). Computer screens are set to 800 pixels wide by 600 pixels tall.. So a image that is 800 pixels by 600 pixels will take up the whole screen.

More detailed explanations of Various File Formats and Resolution

See how to Crop and Resize Your Images

More on GIMP