Pictures disappearing

[Mhyreade]

i have four walls in my house that the pictures on them just randomly fall off. If I'm there at the time I can pick them up and put them back up but most of the time they just disappear. Does anyone know why this might be happening and what to do about it? All other pictures stay put, the walls are made out of basic game materials and 2 that are near a fireplace are more than the required distance from the fire.