PNG can only be smaller for images with a limited palette, which is relatively rare. Clip-art and sketches are compressed nicely, but photos and detailed images in general are HUGE. JPEG is usually better for low-res images and thumbnails, but viewing full size and downloading should give original files, in whatever format they were uploaded. Artist's choice should be respected.

You can't block anyone from seeing your public content, this is silly. Nothing stops them from creating a new account. "Blocks" like this are just a way to annoy others, not something which protects anyone.

What is max resolution here?

Angle brackets are enough: <http://example.com>.

Support for complex queries in the blacklist would be great. But FN supports only simple "A and B and C" queries even on the main page. :(

HTML is supported on many sites. I don't see anything unsafe about it.

While automatic rating sounds good in theory, there may be problems.

* s-media-cache-ak0.pinimg.com/736x/35/9e/00/359e00504ce50c1b8b6101ef985bebdb.jpg - this is gore. But is it explicit, extreme and 18+?

* When you watch lions killing antelopes on National Geographic, it's gore. But kids watch it way before reaching 18 years.

So, if you decide to implement it, please leave an option to override automatic rating.

LOL. If it's a copy, it's a bad copy. ArtStation's navigation (shortcuts, going back) and design (more space for descriptions) are better.