Resolution
The resolution of your display is how many pixels are displayed on your monitor expressed in width by height. Therefore if your display is set to 640x480 you would be able to view an image of 600x400 pixels in its entirety but you would only be able to see part of an image which measured 800x600 pixels and you would have to scroll both horizontally and vertically to see the rest of it.

According to a web survey in October 1996 almost half the users on the internet have a 15" screen or less and approximately 20% of all users are at 640x480 resolution.

The images I have placed on my site are of a size that will always fully display regardless of the viewer's resolution. In terms of layout, however, the pages look better at 800x600 and are still satisfactory at 1024x768 although the images appear somewhat smaller.

This is how the navigation icons on my site appear at these resolutions...

Colour Depth
The number of colours in your display is known as the colour depth. To view photographs satisfactorily you will need to be in at least High Colour mode (16 bit or 65,000 colours). True Colour mode (24/32 bit or 16 million colours) is better. The number of colours you can display at any given resolution is dependant upon the amount of memory on your graphics adapter. With 1MB you can display True Colour at 640x480 only. With 2MB it is possible at 800x600 and with 4MB at 1024x768. There's really no need to use 640x480 these days. Even with a 14" monitor 800x600 is comfortable selecting 'large fonts' from your display if you find the menu bars too small and if you only have 1MB of memory on your card you can still use High Colour mode. The extra space on the desktop is definitely worth it.