Add the following string value to Registry key HKCU\Software\Microsoft\Internet Explorer\Toolbar:
BackBitmapIE5

Then go back and edit the string to the full path of a bitmap file on your hard drive (that you have access to).
For example, C:\WINNT\aluminum.bmp

Remember that the image will be tiled to fit. So, you probably don't want a picture of a person or a pet. However, a brushed metal or psychedelic swirl image gives an interesting effect! It would be interesting to know if this works in later version of Windows OS.