Well this was a fun internet archeology expedition, the model goes by Rena and the photos seem to have originated from a site called Rena's Room in the early 2000's. I'm struggling to pin down if she goes (or went) by another name, but here's one source I found. One of the Reddit posts I stumble across also linked an imgur album that's somehow intact nine years later.
Enh, it's hard to say. The body shapes are certainly similar, but generally I like to have something a little more concrete (like a mole, scar, tattoo, etc or credible written source) to make a match like that.
No clue, I know Lemmy likes webp since it helps cut down on storage costs, but I've never dug into the mechanics of how it's set up. Op's image could also be suffering from repeated compressions as it's been uploaded/ downloaded/ re-encoded and passed around the Internet.
I’ve been making posts using Lemmy’s built-in hosting and they turned out fine quality, so I think the key is to convert to WebP using your own encoder and quality settings and then upload it. If you upload a different format then Lemmy converts to webP at very space saving/low quality settings.