Thanks for the response! I made lots of these changes over a month ago, and I can see on the Google Search Console that the pages in question have been re-indexed several times since then. I even manually requested a re-index of a couple of the pages...

Hundreds of my images used to be in the top 5 or 10 on Google images. In that respect, I wasn't trying to "improve" my rankings but I thought it would be sensible, from a discipline/future-proofing point of view, to change all of my image titles from...