Somehow I subscribed to popular.newsblur.com as a regular feed instead of as a special Newsblur shared feed thingy, such that it appears under my “site stories” rather than “shared stories”. (I suspect maybe I didn’t realise the Newsblur icon was a shortcut to add it as a shared story feed, and I manually added it using browser RSS icon or Newsblur’s normal “+” add feed dialog. Sidenote - would be cool if this dialog had an option for adding shared story feeds or automatically recognised them.)
Anyway the user picture comes through inappropriately gigantic. I feel like this only started happening recently - I thought maybe the feed was broken somehow and the same thing kept getting posted over and over, took me a while to realise it was just the user icon. 
Thanks folks!