I think aldostools added compression to wmm, but I don't know if it's psx and ps2 image files or just one or the other.
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it's a recent edition, so you need to have the latest version of wmm. I'm thinking it's just psx, but it might be ps2 as well. remember: it has to decompress it first before it will play, so it might take a few moments to boot the game.