SarahMK (maybe it's SaraMK) has a set of default painting templates on MTS-- I used hers, and just upped the image size astronomically. Because I hate pixelation, yo. Feel free to use mine as templates if you like, but it helps to keep the guide pics handy so you know which paintings of mine replace which Maxis paintings.
One weird thing is, although the paintings will be rectangular, the textures need to be square-- you'll have to first make sure they're the right proportions for the easel (15x24, so if you're working with an image you want to be 1024 pixels tall, it'll need to be 640 pixels wide) and then skew it so that it's a size SimPE can read properly (1024x1024, 2048x2048... But if you're not as insane as I am, 512x512 would probably be okay).
It'll look like something shot for a TV screen stretched to letterbox size, but it'll work right in-game.
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One weird thing is, although the paintings will be rectangular, the textures need to be square-- you'll have to first make sure they're the right proportions for the easel (15x24, so if you're working with an image you want to be 1024 pixels tall, it'll need to be 640 pixels wide) and then skew it so that it's a size SimPE can read properly (1024x1024, 2048x2048... But if you're not as insane as I am, 512x512 would probably be okay).
It'll look like something shot for a TV screen stretched to letterbox size, but it'll work right in-game.