Do your own. Any picture editor should be able to change the file type of a picture. For wallpaper, you will want .bmp (windows bit map). And if you find a larger pic, you can easily shrink it to 800x600. Best done after you've saved it as a .bmp though. You can expand a smaller one but will lose some clarity. Then save it to the same folder as your existing wallpaper so you can find it easily.
If you don't have a program that will easily do this for you, Irfanview is free and easy to use.
And here is a nice barbel pic that is just the right size.
Wasn't originally but I resized it for you. Just needs conversion to .bmp.