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You can add white bars to the top and bottom of your image to make it square, like i did here: https://www.cgtrader.com/gallery/project/zaz-3 That doesn't change the fact that cgtrader's gallery is retarded though.
yeah adding white or black bars is what came to my mind too. i hope the change it. it's disturbing and wasting time for just posting photos. i wonder why, cause in the model uploading, that's not an issue.