Ok, open up resource hacker and set the file type to all files. Then go to c:\Winows\Resources\Themes\Luna (Where C:\ is the letter of your hard drive windows is installed on, and Windows maybe WINNT)and in there will be a file called Luna.msstyles. Open it and expand bitmaps, then head down to BLUE_TASKBARBACKGROUND_BMP. Expand it.
Open 1033 and you should see a small blue square that looks like the taskbar, but only a small section of it. Right click on 1033 and select replace resource, put whatever file you want in.