@Ghajik
Wait, you used public tank scans and then saved them with a low quality jpg to boot? And because when you saved in png (with RGB, transparency and probably no even compressed) and it got over 100 MB, you decided to save them into jpg with as low quality as possible to the point when you can even see artifacts with a naked eye? Let me facepalm even harder.
And no, it's not mangadex problem. Though since you used upscaled LQ raws that a-zmanga shitting out, there's no point blaming you for them being shit to begin with. Your fault is that you
used them, and then saved into LQ jpg to kill scans even more, instead of using freely available big scans from web-ace, cleaning them properly from jpg artifacts and saving into 8bit/grayscale png.
Biamam haven't uploaded for 4 months
How about contacting them and offer your help with it, so you could learn a thing or two from them? No?