Well, you'd have to read FA's Code of Conduct page for absolute certainty, but as long as you give credit to whoever made it (whether it's official or not) I'm sure you'll be fine.

And I have seen people post pictures on here of various merchandise they've purchased that have been up for a few years, and they've all left links to where they got it from (assuming you bought the stuff online).

I read the rules of conduct but unless I missed it, it wasn't exactly clear.

Well then, if you're very afraid that this could be a problem, I'd recommend contacting an administrator, or use CTRL and F on the CoC page to see if they specifically mention merchandise.

But honestly, I doubt that you can't post pictures of physical things you've purchased- I mean, people repost commissions they receive with no problem as long as they identify the artist and acknowledge them. So if posting merch WOULD be a problem, I'm sure an admin would just ask for you to take it down; I would bet money that it wouldn't get more severe than that.