Improve images scraped from Commishes Portfolio

[faucetlol]

Apparently it's actually possible to scrape images that aren't those godawful upscaled indexed PNG files, been very busy lately but I'll try to get to this at some point if nobody beats me to it.

• https://github.com/mikf/gallery-dl/issues/4935
• https://derpibooru.org/forums/uppers/topics/about-pngs-from-portfolio-commishes-com
• https://discord.com/channels/431908090883997698/460517895420772353/1185341329660715068