These allow you to hold your painting, or the like, onto your wall without putting any holes into the painting, having a professional re-wood it, or what-have-you. They also have a nicely minimal look, and allow airflow behind the picture.
Either print four, if you think your screws and painting match mine, or fix the variables at the top of the .scad file, and re-make the stl. (Note that the file given is for four of them.)
corner_size is the size you want the (non-hypotenuse) sides of the front triangle to be -- basically, an aesthetic choice.
painting_depth is the size of the pocket the painting sits it. I'd give it a bit extra if I were you, so it doesn't scratch up the surface.
wall_size is the size of the bits that aren't otherwise constrained -- I'd put this at about two filament-widths if I were you.
The three "screw" variables specify the size of the screw you will be using to attach to your wall. (I said no holes in the painting. The wall is another matter.) screw_shaft_d is the diameter of your screw shaft. screw_head_d and screw_head_h specify the size of the pocket to counter-sink the head into, so it doesn't mar the back of the painting.
You can't screw in all four corners of the painting; there would be no way to put the painting in. (You could probably modify the design to fix this shortcoming, if you are so inclined.) I screwed in the bottom two, and then blue-tacked the top pair, which don't need to hold much weight anyway; they just keep the painting from swinging forward.