Does anyone know if I can attach a photo to a reminder in things 3 or OmniFocus?
You can in OmniFocus.
This isn’t possible in Things, but there’s a good workaround: https://thesweetsetup.com/using-bear-things-note-attachment-repository/.
The folks at The Sweet Setup are using Bear (as am I, and it works well), but it should work with any similar app that supports URL schemes.
That said, I don’t often have a need to use attachments in my task manager; I’m far more likely to be sending Things a link to an email.