Why not create the thumbnail (any size you want) and then create another
picture either larger or small and use swap picture on rollover of a
transparent giff placed over the picture - just a thought
I have done it with a few of my pictures and it seems to work OK (I don't
have any words on the larger pictures but it would be very simple)
On this one I have just changed the backgrounds on the graphic and made the
photo enlargement slightly smaller than the other example http://www.ukoutdoorpursuits.co.uk/
Good luck
PS - I know "very" little about websites so it really is simple.