Enter Mr. Fork. The perfect utensil for the job.
The fork is nearly perfect. Its got a long handle that will help support the paintings as they as standing upright. The tines can be bent so that the ones in front hold the picture in place, and the ones in back provide stability. If the handle isn't quite at the right angle, its just a quick bend away to make everything perfect.
So with that in mind I set about trying to make one of these. Yes, before you ask, I do have a video of my making one, but lets not get ahead of ourselves. This one is actually the only one I've made to date. I've got a stack of other forks, but have yet to set about bending them. You can see that it really doesn't take very many bends to make a highly effective holder. The only thing to watch out for is that you get the two upward bends of the outside tines as close to the same as possible.
Once you've gotten the outside tines bent, the rest really is just adjustment. You can make the fork sit up higher, or lower, you can adjust up or down the handle. Whatever you want.
But you know what? I don't want to give away too many of my tips here. If you want them all, you are gonna have to watch the video below. Don't worry, its short and painless. LOL.
At any rate, enjoy the picture, enjoy the video, and thank you for the quick read.