UPDATE: I just found this option:

Settings shortcut: Media > Shown options when peeking

That's where a "keep open" option could go, as well as a save button.

A lot of thumbanils have high resolution, and I want to see some detail on them without loading the article just to see it. Especially since mobile web pages most often don't allow zooming anyway.

Another option would be to have a save button you could release your finger over to save the thumbnail.

Even if it is an image post, these buttons could be available since I often don't pay attention to whether a post is an image post or not, and peek the image either way.