Its probably been said before.
Looking a little at the code i found that the links for the solved puzzles go like "squares_01.jpg" when not solved and "squares_solved_01.jpg" when solved.
And when looking at the url of image 5 it is
http://www.oddworld.com/soulstorm/ed/GE3IJNSA.jpg
I've already tried putting it in the place of 7 and 9.
Does GE3IJNSA have something to do in the game?