Just changed a bunch of window backgrounds myself.
1) Depends on what window you want to change the background to. You probably would have gotten help much faster if you had specified that in the first place.
2) SHOULD be one of the following files, unless you've changed something else (a la, installed a mod that changed them somewhere.)
windowelements.dds or
window_elements_background.dds
These files should all be found in the images folder. If they aren't there in your custom UI, copy them over from the default folder before you start modding them.
3) Important things to remember: If the size of these files is way off, EQ2 won't use 'em. windowelements.dds should NOT have mip maps generated for it, but window_elements_background.dds SHOULD have mip maps generated when you save them. If you are using photoshop with the plugin to mod them, you may still need to run them through DDS converter 2, to get them down to the proper size. windowelements should be 1025 KB after you finish editing, and window_elements_background.dds should be 342 KB.
4) Note that windowelements has several possible background textures. Take a good look at the coordinates for the section of the file it's looking at for the particular window you want to mod. In my case, I changed all of the backgrounds to a dark, marble style background.
Hope that helps!