Okay, I got to the bottom of this. I was given a link to 2.4.2 on Aptoide and installed the older version again, but that didn't solve the problem I had. Turns out the image viewer was never broken, but the particular book I was reading had some kind of oddball images that couldn't be viewed that way. I re-installed 2.5.0 and found I could still zoom images in some older books I had that I knew worked before.
Conclusion: Nothing wrong with OMC 2.5.0