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SMG3's Painting is a minor villain that appears in SM64 Bloopers: SMG3's Gauntlet of Gloom.
It is just a painting of SMG3 set with a flowery background.
Despite being a painting, it is actually a security drone which will destroy intruders to SMG3's gauntlet. It does so by lobbing bombs at them, which it did by throwing them at Mario while he was running past him.
It can also survive its own bombs, as shown later in the blooper that although it was hit by its own reflected bomb's blast, it was seen as fine when Bowser also went through the same hallway SMG4 and Mario was just in.
Apparently, it does not appear anywhere after the episode nor it is mentioned by anyone else. Therefore, it can be safely assumed that it was destroyed by the exploding gauntlet when SMG3 activated the self-destruct command.