Create or find a .BMP file you wish to use. Navigate to:

HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Toolbar

Add Value name BackBitmap, a type REG_SZ value, and set the string to the full path to your bitmap (c:\winnt\jsi_bg.bmp).

Restart IE 4.x.

NOTE: IE5 also has BackBitmapIE5.