A number of screen resolutions are listed below. Pick the one that is correct for your personal computer and click on the bar to reveal the pictures available in that size.
Not all pictures are in all resolutions. Though not many people use 800 x 600 anymore there are quite a few in that size. There are less in some of the more recent resolutions.
If you don't see the image you like in the size you need take a look at the next size up and download that. You will get a better effect by scaling a large image down to fit your screen than trying to stretch an image to a larger size.