con-artist master manipulator

Well, what does "con-artist master manipulator" exactly mean?

No. You could read that as con artist AND master manipulator. You wouldn't say "con artist master".

oristarA wrote:

Well, what does "con-artist master manipulator" exactly mean?

It means he is (1) a con-artist (confidence trickster) and (2) a master manipulator. These are words and phrases readily found in a good dictionary.

But the grammatical structure of "con-artist master manipulator" is or appears to be "master manipulator of con artist."

Is a comma needed: "con-artist, master manipulator"?

You are suggesting he is a master manipulator of con artists? Unlikely, but go ahead if that's what you want.

But the grammatical structure of "con-artist master manipulator" is or appears to be "master manipulator of con artist."

oristarA wrote:

But the grammatical structure of "con-artist master manipulator" is or appears to be "master manipulator of con artist."

The structure may seem that way, especially to a non-native speaker. A native speaker might notice an ambiguity but would very likely resolve it in favour of the more natural interpretation (that con artist and master manipulator are separate), especially in view of the context and his/her general knowledge. Compare Raffles the Gentleman Thief and Jack the Giant Killer. The writer is probably not a professional - people who superimpose messages on photographs for the Web seldom are.