Now it Registers Properly
I thought I had properly vetted the code and it was doing what I wanted. But, nope, the API was never using the ID field, and used the Name field as the ID. I don't know why Minetest allowed Capitalized node IDs. This has been fixed, and should work excellently now. Unfortunately, all previously registed paintings will be lost. My apologies for this. I have added around 20ish paintings to the Gallery below to help lessen the blow.
A simple paintings library that lets you easily add 1x1, 1x2, 2x1, 2x2, 3x2, 3x3, 4x2, 4x3, and 4x4 paintings. The API has a seperate registration for each painting size to keep things simple. All your mod has to have is a painting texture of the right size and shape and paintings_lib.register1x1("painting_name", "Painting Description", "painting_texture.png^painting_frame_texture.png") for specific examples, just look at the paintings.lua of this or paintings_gallery. Want to actually get some paintings without having to register them yourself? the Version II is better in my opinion, but both are here.
Temp link until it gets approved.