What does a picture falling of the wall mean?

Perhaps you are experiencing an earthquake!

Rush to open areas away from buildings and braking glass, or if indoors go under door frames or strong tables.

On the other hand, perhaps the nail holding it up has just loosened over time and it fell off.

Not all naturally occurring phenomenon can or should be interpreted as signs or omens. That said there is a widely held superstition that says a picture falling from the wall (with no reason) is a bad omen, possibly of an upcoming death in the household. - however it is said the glass must break for the omen to be true.