Does this happen for all custom paintings?
I haven't seen this happen
yet in my pre-OFB game. I have a family who has moved 2 or 3 times, and every time the paintings (placed in the father's inventory) are fine. One of them was a portrait of the father's dead son, and it also transported successfully. Do you use any hacks that may be affecting the paintings? And this may be irrelevant, but whenever my Sim paints a portrait of a family member, I always select the "paint Landscape" rather than "paint Portrait" option, simply because it doesn't involve the subject having to lie around for hours.
Max