What will happen is that people with the smaller screens, their browsers will show the image as is but will have scrolling bars for virticle and horizontal scrolling. If you ask most people, they find that to be a pain in the butt when it comes to viewing an image they would like to enjoy in its' entirety. So most people upload images that are large enough to offer good clear resolution but small enough to fit entirely in most of todays screen sizes, that being within either 640x480 and 800x600 range. The most popular screen sizes today are 1280x1024, 1024x768 and 800x600. This being the case, logic dictates that 800x600 (4:3 ratio) should reasonably be the largest image size for comfortable viewing. BTW, good luck and enjoy your new rig.