This is the sort of discussion where a little bit of data would go a long way :-). In particular, Joanna you should give us:
1. your current screen resolution (right click on desktop, choose display preferences (I think) and go to Settings).
2. a link to the image you are trying to set as your background.
As previously mentioned, it’s unlikely that random paintings are going to match your screen resolution setting (though many wallpapers are created to match the standard set of screen resolutions), so the image will need to be cropped, tiled, scaled, centred, or stretched by your system. Which one of these is the right way to go depends a bit on the answers to #1 and #2.
—ravi