I don’t have many general graphics articles on the site and this one was the very first one that I wrote. There are many different uses to which graphics can be put and the appropriate resolution for the image will be different on that use.
Determining what Resolution is Appropriate

Early monitors supported graphics modes that allowed for far lower screen resolutions than the 640 by 480 of VGA mode. Some even allowed for fewer than the 16 colours that VGA mode is guaranteed to support as a bare minimum. By the time that Windows came into common use VGA mode was the accepted minimum [...]