I regularly scan my bills as I receive them and store them on my hard drive.  The 
feature I would like to have for GNUCash is to be able to associate a file (or more 
generally a url) to a register entry. 
 
Quicken had (if you had PaperPort installed) a similar concept.  There was an 
entry in the register 'edit' menu that specified 'attach file'.  Once you attached the 
file Quicken would show a graphic that looked like a ^.  If you clicked on this it 
would bring up an image view dialog. 
 
My idea is similar except I don't see a need to have a GNU Cash based viewer, 
just use whatever viewer for that image file that is installed/configured on the 
machine.  Thus, all you would have to provide is a way to attach/detach the 
image/url to the register entry and a button in the register (only visible when an 
image is attached?)