Requested feature overview description.
The post thumbnail would indicate (via "number of favorites" counter turning from white to a distinct-yet-still-legible color, glow around border, etc.) whether the current user has favorited that post.

Why would it be useful?
It would allow users to tell whether or not they have favorited a post without having to open the post.
A minor convenience, but since it's a feature on many booru websites, my browsing habits have come to rely on this feature.

What part(s) of the site page(s) are affected?
Posts

P.S.
I've searched the forums gazed long and hard at the help section and haven't seen this mentioned anywhere.
If this has already been addressed then please enlighten me.

Sounds nice, but it's worth noting that you can include or exclude favorited pictures from searches with favoritedby:name, adding a - before the meta tag if you want to exclude. I'm not suggesting against this, just pointing out a seldomly-known feature.

siral_exan said:
Sounds nice, but it's worth noting that you can include or exclude favorited pictures from searches with favoritedby:name, adding a - before the meta tag if you want to exclude. I'm not suggesting against this, just pointing out a seldomly-known feature.

Yeah I use that feature if I've already favorited lots of images in the results of a given query and I just want to see images I haven't favorited, but usually when browsing I just like knowing if I've already favorited an image at a glance.