When you recreate the game.zip file after modifying the image you have to make sure that it has the same internal folder structure as the original. Often compression tools will create a zip file that has a single folder inside, which then contains all the other files. This prevents the engine from finding the files, so make sure it's not what's going on in your case.
Turns out I did zip the folder instead of the contents, but even after fixing that I couldn't get it to work. Ended up downloading a clean copy of the game and redid the unzipping/rezipping process, and that did the trick! Sometimes starting over is easiest.
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