Fair comments, and I appreciate there are no fast rules.
I guess my standpoint in general is dofollow everything unless there is cause not to. With image credits I see them as natural references, just as I would when quoting the source of an article, and therefore there's no reason to nofollow.
Ultimately up to the site owner, unless of course there are specific terms and conditions attached to the image in which case either follow them and have the image or choose another image.