The banner for my website is a little off

hi

I created a banner for my forum earlier today, but for other users it seems to be a little off for them (saying the banner stretches too far out to the right, and doesn't fit the right side of the page). However on my screen, it fits just fine. I think this may be happening due to their screen size. Is there any way to fix this, so that the banner will look and fit the same for all users, despite screen size?

This is the link to the site, so you can see if the banner may be off for you:
http://explicitwarfareca.simplygaming.org/forum.htm

I think I could try recreating the banner but in a smaller size, but I'm not sure if it would fit for all users yet.
Any suggestions?

I just checked your forum and I saw the banner and is to big so it goes out on the right side.

Ok, I think I'll re do it and make it a smaller size.

Are suggested sizes?
i think I used something like 1000x300...