I thought I'd put a uke or two on Reverb to sell them, but I discovered that Reverb requires square photos. Look at the shape of a ukulele! "Square" does not apply. I'll have to take new pictures.
I tried posting some, and they said they weren't right. I think they display them square, so if I posted a rectangular image, I would get a warning. I think I have that uke sold on Fleamarket, though.
Nothing but good experiences buying and selling on Reverb. In fact, selling old instruments I didn't play any more funded my epically bad UAS when I first started playing ukulele 2 years ago. I don't recall having any issues posting photos. Most computers have basic photo editing software on them these days, and you can just crop photos to fit the parameters.