Originally found at http://www.jbrowse.com/text/bmbk.shtml.
Apply the patch to vim source and compile. This results in a vim with FEAT_BMBG (bitmap background) enabled. The output of :echo has("bmbg") should be '1', and :version should include '+bmbk' in its output.
When the GUI is initialized, gvim will look for a file in bitmaps/background.bmp under the vim runtime directory (e.g. c:\vim\vim63\runtime). Yes, the name of the file is hardcoded because this is Not A Real Patch, yet.
There are plans to improve the patch such that it is more user-friendly and complete. This is currently a patch for win32 only.