Actually, if the image you are using is 240x260, when you set it as desktop wallpaper the image will get cropped. If you are creating your own image to use as wallpaper and you don't want it cropped, make it 240x185 (the "title bar" covers about a 10-pixel-high area, but a tiny bit of the image is still exposed on both sides of it -- if you want your image to fill those parts too, make it 240x195).
If you'd like to try a test image (a picture of the sky I took last week), point your BB browser to
http://johnstoneham.com/images/sky.jpg
Here is a preview of it: