I've noticed that my screenshots are small but cover image is normal. When I took a look at image links i realized that cover has
this:

and screenshot has this:

That's literally how fastpic made their host.

Try uploading larger image manually (via browser) - you will get a warning from them, that larger images don't support direct link and the only way is to use thumbnail link.

Report to them to change it.

But in case of the cover image works ok, this only happens with screenshots. Their new upload page returns both thumb and big image. But direct link dos not work.
It's interesting that cover image with creator works fine but preview does not.

Cover image is smaller (they allow big link for smaller images).

Preview image is larger (they don't allow big link for larger images).

You can try it manually via browser. They do not return big link for larger images (make sure to try to larger image).

Attaching screenshot (it's literally their error and that's how they work).

Contact them or use another image host.