I'll be honest and say that while I like the conceit and art of this game, all of the puzzles have thoroughly annoyed me so far. They are very obtuse and are worsened by the limited game design (highlighting interactable objects/areas with an outline when hovering over them would drastically improve it). I've either had to luck my way into correct combinations or look to posts so I can cheat, but the one that nobody has (properly) described the solution to is the fucking painting. I don't get how I'm supposed to click these corners. Can someone please walk me through how to solve it so I can move on?

If you found all the barcodes(5) and scanned them with the ipad, then just click the corner in the number order of the scans. 

Exactly, I'm at the same spot and so confused. I also don't know where barcode #4 is 

You need all five barcodes, because one corner is repeated. you can get the last one by having the ipad selected and in "bar-code mode"