See this:

http://mendhak.worldjunkie.com

Notice the black background (with the frog on a branch).

Now this image is 800*600, I also have a 1024 resolution of the same image. How can I specify the appropriate background for people with different resolutions?

Also, I understand that for 800*600, I shouldn't put an 800*600 image, but it should be a little smaller... what should this dimension be? and for 1024?

Thanks.