Is any convention to tell the propeller rotation direction of an airplane?
When you say "clockwise" or "counter-clockwise" it is generally accepted that this would be the view as if you were standing behind the aircraft. Unless it is otherwise stated, this is the convention that you can assume. 

If you need a way to tell by looking at the prop, the thicker part is the leading edge. In the first photo you posted, this would rotate counter-clockwise from the perspective of the photographer, or clockwise from the perspective of behind the aircraft. This is, by convention, a clockwise rotating propeller.