try this script from pone
[WS] Google Images Cover Search
EDIT
Failing that try this, its a quick dirty fix but seeing as many songs dont have "&" in the title I just changed the search function around.
for all sizes
http://www.google.de/images?q="%title% %artist%"
for 800x600px or larger
http://www.google.de/images?q="%title% %artist%"&hl=en&biw=1236&bih=1372&tbs=isch:1,isz:lt,islt:svga&source=lnt&sa=X&ei=-u1wTZTpPMK1hAeb0qkx&ved=0CAoQpwUoBA