This is a very late reaction to this post, I know. But I just found a good solution for me and I just wanted to share that. Possibly people will still search for a solution, just like I did.
For me the Moveobjects cheat works only 10% of the time or so, so no real luck with that.
The best thing is probably what MoonsAreBlue said in the other tread about invisible paintings:
The best thing to do is when your sim completes a painting, go to buy mode and put it in your family inventory. That way, if you place it on the wall and it disappears while is buy mode, you can still hit undo.
But if you're as unlucky as me and you have put all paintings in your painters inventory already, that doesn't work
because you cannot move something from the personal inventory directly to the family inventory...
After some puzzling I found out that when you move the paintings to the inventory of another sim of that household, your problem is solved. Just drag the paintings one by one to the other sim and let go when you see the sim light up. From that other sims' inventory I was able to move all paintings to the wall and didn't loose any!
Of course, now you
can place them in your family inventory if you want.
I hope to have helped somebody with this!