When I mine blocks to the right side of a large painting, the painting disappears after a few seconds. For example, consider the following (X is blank wall, P is painting, best viewed in a monospacing font such as Courier):

XXXXXX
XPPPXX
XPPPXX
XPPPXX

Remove two blocks at the right edge of the painting, and the painting will soon disappear, creating a blank wall with a door, like this:

XXXXXX
XXXXXX
XXXX X
XXXX X

It will then be impossible to place a large painting whose right edge touches the opening. However, if a door is installed into the opening, as follows, then a large painting can be put there:

XXXXX
XXXXX
XXXDX
XXXDX