Using Qt's inbuilt image codecs, it does not seem to save the alpha channel. This causes the LZMA compression tests to give LZMA an unfair advantage with datasets which contains transparency, and I have a rather large one which reduces cgCompress efficiency from 25% to 5% average over all datasets, as just using LZMA should be more efficient for those images. However this is not fair, as it does not include the alpha channel.
It does however seem that for those images, LZMA is best, and even deflate is better than WebP. PNG ends up with about same size as deflate.
Since it is only the evaluation which would use BMP, just fixing it there would be good enough. We could just write a simple stream manually, instead of using BMP. We could even try to interleave it, to see if that improves matters. It needs to be fixed in the end though, but not needed for this issue.