I don't think it's stealing to screen shot that super-low-res 'nintendo-64 resolution' finisher pic with their waterstamp across it. They want to give you access to preview that, it feels fine to screenshot it. I didn't make them go out there and take my photo.

If I didn't ask them to take my pic, and the low-res photos are out in public, I should be able to screengrab it if they intentionally put it up there in public.

Similarly, if they intentionally put the entire hi-res version of it up in public on a public link, watermark or not, I also believe I have the right to screengrab it, as the entire world can do the same.

Now it's stealing if I hack into their computers and take the hi-res photos that they are charging money for. I would also not download them if they inadvertently left the images public-accessible for a short time but clearly didn't mean to, that would also be not cool.

I also think the photog would have grounds to sue if I was some high-profile for-profit blogger that makes money of my FB posts, and I used that low-res image an a post of mine. Then I'm profiting off their work, and that seems like grounds of infringement. But that's a different issue than Mr. Joe AGer posting on his FB to his friends about the local triathlon.