The density of pixels per inch (ppi) is called image resolution. Although there is variation between monitors, the rule of thumb is that screen resolution is 72 ppi. This means that all images used for the Web should have an image resolution of 72 ppi.

Why not use higher resolution? More pixels per inch means a higher quality image, right? Well, only if your are displaying your images on something that can show all those extra pixels. On the Web, your reader's monitor is the limiting factor. You could send more pixels to display, but they'd just be ignored. Higher resolutions are simply bandwidth wasters, needlessly inflating file size with no visual advantage whatsoever.

However, it's different if you intend to print the image, rather than display it on a Web page. Printers typically produce from 300 to 600 dots per inch, or dpi. (Dots and pixels are the same concept, different terms.)

By the way, don't confuse the terms image resolution and color resolution. Color resolution, also know as bit depth, refers to the number of possible colors in a digital image file.