Try freezing the picture - sometimes this works. Also, pictures usually get stuck due to moisture. Either there might have been some moisture left on the glass if you cleaned it before putting the picture in, or else it could have been sprayed with glass cleaner while the picture was in it and some got into the little cracks between the glass and the frame.
This happened to me before, too, and now I always just use a Windex wipe or paper towel with Windex sprayed on it to clean my pictures.
Good Luck!