I disagree that it's all that unintuitive - some users will misunderstand no matter what. The hover text you placed should help.
The real problem lies with the name "Hidden Gems" on the front page. It's not readily apparent that users have direct control over this, hence the disconnect.
If that section were renamed "Pinned by users" or "Community Recommendations" or "Featured Images" that would make the functionality more obvious, albeit at the expense of obscuring the intent of bringing exposure to lesser known models.
Then again, the criteria for what images are pinnable isn't obvious anyway.
I think Green Tea is on the right track. Rename the section "pinned by users" and leave button on images as is.
So I searched what "Pin To Front Page" means and found this thread. Good to know. I was not aware that it is tied to the Hidden Gems section on the front page.
I think that you should rename it. Most people know what Hidden Gems is because it's prominent on the front page. For me, if the title would've said something like "Vote for Hidden Gem" or "Mark Hidden Gem," then I would've known what it was without searching the forums.
Kind of a small detail but it was pointed out here that the "Pin to front page" button is not intuitive as to what it does, and that it is connected to the "hidden gems" section in particular. Can anyone come up with better text for it?
The main constraint is that I don't want it to be longer than its current length because I want this button, the Report button, and Subscribe buttons to come in under 300px to be in one row on small screens.
One other option I thought of is "Mark hidden gem" but this fails in communicating what a "hidden gem" is and what the actual practical effect of clicking the button is.
I did add hover text to it just now so maybe that will help.