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IIRC, it's for under 2000x2000. There's no way of knowing the DPI. (Many formats do have a DPI field internally, but it's rarely set correctly.)

I don't think it's helpful to persuade people to upload less; it's not like the board is being flooded with terrible images. It seems better for people to be a little too liberal than too conservative--if a post is poor, just delete it and they should get the message.