The getting started pages include a video tutorial called “How to Take Identifiable Photos”, but the three tips are: get closer, shoot multiple angles, and take photos that are sharp and in focus. Maybe the video could add “give a sense of scale” or something like that? I don’t know if @tiwane would be open to adding more to the video though, sometimes it’s better to keep it simple.
There’s also a discussion going on how to take photos of particular taxa, but I’m not sure how wide of an audience that’s reaching and scat isn’t exactly taxon specific.
I occasionally ID skeletal remains, and measurements are sometimes the only way to distinguish between similar taxa. On a case-by-case basis, I ask the observer to please try to include a scale in skeleton photos. Sometimes they still have access to the skeleton and can even go back and add photos. Sometimes you just have to accept that a species ID is impossible.