<rant>

I must say that they developers kind of dropped the ball with the painting puzzle. The painting doesn't have easily distinguishable colours and the image's layout is kind of weird in general. Also, the blocks slide slowly, which causes extra annoyance when you end up machinegunning the puzzle in frustration.

Pandora Directive had one of these; I wish they didn't think they had to include EVERY element from the previous games. These things are even difficult for a computer to solve.

Most of all, it's annoyingly late in the game, so players can waste time with the infuriating puzzle, look for a video solution, restart in casual mode or just be put off by the activity book bullcrap and go play something else instead.

</rant>

As someone who absolutely hates slider puzzles in adventure games, I don't hate this one. The main reason is that it's probably the most believable slider puzzle in any adventure game, in that it doesn't actually do anything by itself. I can't stand it when slider puzzles are used as "locks" for doors - who the hell locks a door with a slider puzzle? Has that ever happened anywhere throughout all existence? Every time they exist they're so very obviously a puzzle for the sake of a puzzle that they ruin the immersion somewhat. Except MAYBE this time, although it's still not likely that someone would use a slider puzzle for this purpose either.

The second reason I don't hate this one as much is because it's pretty small (therefore quicker/easier) and you don't have to actually complete it fully to get the use out of it. Normally sliders potentially drive me nuts (depending on how my brain is working that day), and they ALWAYS feel like busywork as opposed to a challenge. I've never needed a solution for one though (since for me looking up a solution and then following it would probably take longer than just doing it).

BTW the following may help you (although you've probably solved it by now):

*** SPOILER ***

I'm sure you've managed to complete it by now, but you don't actually need to solve all of it. You actually just need to solve enough of it to discover the important thing it contains.

*** BIGGER SPOILER ***

This almost annoyed me more than the puzzle itself, but it contains three numbers which I completely missed even after solving it! Even once I knew they were there I had to angle my mouse many times to be able to read them. What clued me off about it is Tex mentioning something about it when you discover the safe (which by the way contains a picture that's apparently the same as this one - so there's a reference pic for you).