Large paintings can only be placed on the edge of blocks.
To do this
1. Create a 4x4 square
2.Try to place paintings to get the largest
3.Go to the bottom left corner(the 2x2 corner) and place the paintings until you get the large which should hang off the edge
P.s I put barrier blocks to prove I wasn't lying
If this is already fixed I apologize
Please attach screenshots of the painting from the front and back, without a resource pack.