Could someone explain to me how to add custom images to the paintings? I am so sick of the same ones over and over...
If you want to REPLACE the standard paintings you can copy and modify the paintings_kristoffer_zetterstrand.png/kz.png [not sure which name is in the current version] file (it may be a good idea to keep a copy of the original — just in case).
These days this needs to be added back in as a resource pack. [I haven't changed the paintings in the last couple of additions so you'll need to search for exactly what this involves.]
If you want to ADD to the standard paintings you will need to find (or write) a mod that will patch to change/redirect the vanilla painting code to something new.
THIS documents a very specific workaround by dinizintheoven a few years back…
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.