The iPhone and I believe also the Android app cache all your pictures from your observations (in addition to saving the photos on your photo roll). For casual users that’s no big deal but for ‘power users’ the app can quickly swell to several gigabites in size. Currently the only way to clear the cache is to sign out then back in, or delete and re-install the app. This works OK once you know the trick, but causes confusing for new users. Also, if you are in the field and can’t upload first, you can’t do this without losing un-uploaded observations.
I personally don’t have any use for the caching, and would just turn it off if there were an option - the photo is also saved on the photo roll and can be re-downloaded. However, a button to clear the photo cache, or an upper limit beyond which it deletes older photos from the cache, could each solve the problem as well. It should work when not in cell reception, for those cases you unexpectedly exceed your phone’s storage on an extra productive field day.