Sorry about the usability issues. Unfortunately it's hard to be
equally easy to use for everyone, since everyone has different needs
and expectations. We did hire a designer this spring, though, and
improving usability is pretty much her mandate, so hopefully things
will be improving this year (though she's mostly focused on mobile).
Charlie's instructions for deleting a photo should work. If you're
using the iPhone app, you should be able to edit an observation, tap
the image, and tap the trash icon in the lower left, then sync your
observations.