I love when it tells you it can't do something and then does it anyway.
DreamySweet
The pending thing just means that it hasn't been able to update the subscriber count on fedia yet. On your instance, you're properly subscribed. New posts should show up as they get posted but old posts might take a while.
It's not pointless, it's so they can track you.
what a misnomer too
It's crazy how many people think "incognito mode" prevents people from seeing what websites they are visiting.
I found it on my instance by searching for https://fedia.io/m/firefox
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You might have copied the album link instead of the image. Imgur hides the direct image link so you have to right click the image to get it now.
I think you can edit the original post and change the url to be the imgur image's url. https://i.imgur.com/FQLlDC7.png
In that post, I typed what that icon generates but used the image's url on imgur. 
If I click the icon and upload it, this happens:

My bad, sometimes I am blind to usernames.
Maybe it's an issue with your instance?
OP didn't post an image, they posted a link to the page that hosts the image.
A few of the older posts from there eventually appeared though.
No. Work smarter, not harder.