If you use Google image search it's highly likely that you will end up with a copyrighted image. Try Microsoft's clipart site - their images are all royalty free.
http://office.microsoft.com/en-gb/clipart/default.aspx
Do a search for 'clouds' and use the drop down menu in the search box to choose 'photos'.
If you use a high quality image in the first place you will not need to worry about the dimensions of the photo as you should not notice the tiling. Although Amanada's advice will work per se, it would mean that
all of your web pages would need to be exactly the same length.