[Etshy]

Can't say if the webP usage is new or not.
but the hash part is the same in the thumbnail and in the original image, so the only difficult thing is the subdomain part, I guess.
I made a tapermonkey (dirty) script to try some things and download comics.

I dunno how Jdownloader works, but if it can load/execute javascript it's quite easy to get the subdomain (there is function in the common.js from hitomi.la for that.
if it can't , converting these functions should lead to a fix I guess.

Edit: Btw I don't know how hitomi itself download their own comics, but after few tests my dirty script seems to be about 60% more efficient ...