Use a photo editing program such as "legal" version of PhotoShop CS3... and [Edit Image Size] to exactly fit your background to... ___ 640px by 480px .. hope that helps, I have great clarity on my wallpapers.
What him said! I prefer paint.net for little things such as that if you're in Windows. Will need .net framework from Windows Updates though. Otherwise Gimp will work well, Windows or Linux.
There is an app you can download from Faststone that is free, legal and an excellent view, resize and (minor) manipulation utility. I've used various versions for years and it works very well. There are several other photo utilities as well. Regards,