In general, this site avoids rehosting whole comics that are on their own websites. If I just wanted to upload a single panel from the comic, would that be acceptable, or would it count as a cropped image and get deleted?

There was a post a while back (post #1800174) of a panel from Las Lindas (https://laslindas.kemono.cafe/comic/ll0558/), with the background replaced with a transparent background. I found a copy of the posted image, and it's actually larger than the panel shown in the comic, has a transparent background, and includes detail that was cropped for the comic page. The upload was deleted for being cropped. I don't know what the original source was for the larger size.

Aside from that post, I was wondering what the general rules are for individual comic panels.

These would count as low quality/effort edit of the source material, so these would be deleted on that basis. Even if the source material is not accepted on the site, doesn't change how edits of them are handled.
That specific post almost looks like it was upscaled from the source material, which is not allowed here either.

The upload guidelines say that webcomics are allowed, as long as it's not the whole comic and just a small handful of pages. It sounds like the intention is to avoid hosting entire webcomic archives here (and to not steal their traffic).

I'm mainly asking about the cropping aspect because there are some Las Lindas pages that I'd like to keep in my favorites, but it's just a specific panel in the page that I'm interested in.

If it were zoomed into just the face, I could see that being called a reaction image, but even then you aren't guaranteed to get it approved.

The comic panel I was wanting to upload is the last panel in this page: https://laslindas.kemono.cafe/comic/standing-ground/

On the subject of the deleted post I found, I don't know how to tell if it's an upscale or not. But as I mentioned it's larger than the comic panel and includes portions that were cropped out for the comic itself. Maybe it came from Chalo himself, or otherwise it may have been a very high-quality redraw of the missing areas.