I'm thinking these pictures really do need to include a field for date taken (automatically filled in with values from the photo metadata would be great), location, and generic caption for more info. Location information maybe should have options to select from like user's garden, other private garden or locale, and public locale. I understand the concern for privacy in regards to stripping photo gps metadata, but it could be used for public location photos at the user's discretion. Having a standardized searchable location like country, state, and zone would be great too. The ability to search the photos by location and zone would be great for people looking for inspiration and plant combination ideas appropriate to their zone. It might even make for nice feature articles to showcase all the users own gardens from a rotating featured location. That might show off the diversity of the site's users too.
I have sometimes included some of that info in the discussion area like
@bonehead suggested, but it seems a little odd to make a forum post just to add a simple caption to the photo. Since it's at the bottom of the page, I doubt many people even scroll down to notice it either.