Is that with the browser of your mobile/tablet or of the desktop computer?
On my mobile I can reproduce (for long time). My solution is to do a long-click on the picture and open it in a new tab. There I can zoom in and look at, what I want to see.
Perhaps in my case it’s a handling issue. I have figured out, that when I tap at the picture (it zooms somewhere), stay tapped and move the finger, the zoomed part of the picture moves with my finger. Not helpfull at all, but I assume it’s intended. :(
Using the phone, browsing the iNat webpage (browsers duckduckgo / chrome) in compare mode. Zooming into the observation fotos, to see details, never worked for me either.
My workaround is to look at the observations fotos first, and then open compare. Each time closing compare mode, when i have to take a second or third look at the observation fotos.
I’m not sure if this is a bug or not, but this is how I experience the identify function as well. And would LOVE to be able to lock the zoom screen so that it’s simpler to compare to suggestions without the subject of the observation getting lost.