Every photographer has been confused by the subject of digital image sizes - pixels, resolution, ppi, dpi - what are they and how on earth do they relate to each other? Bob Atkins has a handy guide which answers all of these questions:

“The way you control how large an image appears on someone’s monitor screen when viewing your images on the web is by changing the height and width in pixels If your original image is 1600x1200 pixels it will probably be too large to see all at once on 95% of the video monitors out there. It will also be slow to load since it will be a large file. If you want someone using an 800x600 display to be able to see your image clearly, you need to change the size to, say, 600x400 pixels (remember the browser window is smaller than the full monitor display).”

Website: Bob Atkins - Digital Image Resizing