When the Doctors were in the painting, when they unfroze, everything else came alive and actually exists, as proven when the Dalek said "exterminate" and crashed through the painting. Since the painting still exists, can't the Doctor go back in and rescue a few living Timelords?
Most likely, the Doctors used the same technology that the Zygons used to insert themselves into the already existing painting, giving them a freedom of movement that anyone else in the painting wouldn't have.
The painting isn't a window into another time like, say, the portals on the SS Madame du Pompadour. It's not something that people as a rule can just pop in and out of at will. It's a frozen moment in time, but it's still just a copy of that frozen moment. Photorealism taken to the extreme.
Imagine painting a picture on the inside of a box. You could get in the box after you're done, have the box taken somewhere, and then get out of the box, but anything that's painted on the inside of the box is still just a painting; you can't take it out with you.
Or, rather, you could, with a pair of scissors (or a set of sonic screwdrivers), but what you came out with would still be a copy and probably wouldn't survive. It wasn't the sonics that killed that Dalek, it was being ripped from the painting.