That’s a valid point about the thumb, but the pic does look palm out? The positioning is awkward since he’s walking along in the same direction and talking over his shoulder.

Season 1 TNG was a bit rough - I’d agree with other posters here that it’s either a mistake or Jonathan Frakes having trouble making the gesture.

It is a Vulcan salute: https://en.m.wikipedia.org/wiki/Vulcan_salute

I don’t recall why Riker would have been using one in that context, though, unless TNG was trying to establish the gesture had expanded in use across the Federation.

EDIT: also, this is in S01E15 “11001001”, not E14.