I meant more like scanned comics and manga, stuff that wasn't originally intended for the web. All the websites I have found for scanned mange don't have a scan per screen that you can 1 click through as you read and generally have to be scrolled down to view them one page at a time. That and as you mentioned the site hosting having lag and errors and sometimes ads makes them not entirely ideal for easy reading.
I'd image 1 click per page viewing on Renpy would be super convenient by comparison.
When I said "webcomics", I was using it as a blanket term for anything visual that is then hosted on the web; I meant manga and comics, as well.
If I'm not mistaken, some years back there was a program like that, but I didn't remember it until now, and obviously it's name escapes me. Downloading comics shouldn't be too hard though. A lot of sites allow you to download them, and you should have an image program that can easily swap through them. Although, admittedly I haven't been to a "normal" manga site in ages, and I was mostly referring to the... other kinds of manga.
I'm pretty sure there's programs for stuff like Kindles that do that. So getting the same thing, or something similar, on an actual computer shouldn't be too hard. Most of the times your only going to read a manga/comic once though. So, unless your going to binge a whole series, seems pointless. Even if it were a whole series, you would have to Renpy/Download the entire thing, which would take up a lot of time, even with the aid of a program. Unless someone has already pre-archived it all for you.
I imagine there's an audience for that, if you hosted a website of archived comics, and had a program that unpacked/read them for you. The legality of that is something else entirely, though.