You set an image that is one resolution, to be the background of another-resolution-ed desktop... And this surprises you how? And more importantly, what kind of fix can you possibly conceive of, beyond putting in a different picture?

N900 desktops are 800 by 480 pixels. Either 'stretch' or resize the picture yourself so that it's to your liking, or crop the part you want, so that when it's stretched into the 800 x 480 resolution, it's still the same proportions.

Or, you know, if you can, take/get another picture of the same thing that's 800 x 480 pixels in size, and use that