Why Open Access is not the same as Public Domain

I recently had a conversation with a university colleague which went something along the lines of “You work with Open Access content.  If an image is available for free on the internet without a copyright symbol or watermark it’s free to use, right?”  Admittedly part of my brain wanted to run away screaming from the assumptions implicit in the question, but after thinking about it a little, thought it would be more helpful to summarise it here.