While walking along the river a couple of days ago, I noticed a fairly large dead fish floating upside down. I stopped to take a shot (as you would), but it wasn't until I took a closer look that I realised a dragonfly was perched (sorry for the pun) on the upraised fin. I'd seen one a couple of months ago and managed to get a close-up.
Turns out it's not a dragonfly, but a damselfly. Specifically, a Common Blue Damselfly, or Bluet (Enallagma cyathigerum). I also learned that damselflies usually have their wings up, like a butterfly, and dragonflies usually have their wings spread, like moths.