These hand signs are used often in catholic art.
(1) The pictures in the link are, forgive me, incredibly silly. To post pictures of people waving or holding their hands up
or even clapping, and to accuse such gestures of having some sort of secret connotation with regard to an ancient mystery religion, is to stretch credibility beyond what a reasonable person could accept. Hold up your own hand and see if your pinky doesn’t naturally drift away from your other fingers.
(2) It would be easiest, I think, if you found specific examples of art in regard to which you have questions. Off the top of my head, I would imagine a similar hand gesture to this is used when depicting the Lord Jesus, a Saint, a Bishop, or any number of other individuals engaged in giving a blessing to the viewer of the painting.
(3) Art is by nature incredibly symbolic. Be very, very careful when people insist that art has some sort of a symbolic meaning which the artist privately wanted to hide in plain sight. The fact is that anyone could find almost any meaning in something that is symbolic, and the potential to project erroneous interpretations is vast. The symbols in art often have their own context based on the school of art being employed, and to be ignorant of this context can lead to a nonsense understanding.