Hi Bimmer,
First, let me say
welcome to RetouchPro!
You ask a very interesting question. I don't think there is a "formal" definition of Photo-Art. I've been trying to write up
my definition for the past 20 minutes and am having a difficult time putting my thoughts into words. So, I'll keep thinking about it and try to write something that makes sense later.
In general though, Photo-Art uses a photograph as a starting point and branches out to create a new "interpretation" of the photo. I view the "just a photo" as the starting point, though even that gets messy when you think about retouching a photo to improve the color balance/saturation and sharpness. I would still consider that "just a photo", but taken to extremes, I might consider it "Photo-Art". As far as "just art", I think that encompasses a whole range of mediums, so both "just photos" and "Photo-Art" fall under the category "art" to me.
If I try to say anything more, I'm going to confuse myself.
Jeanie