Ranma in the manga uses a lot of hand gestures more so than the rest of the cast combined. He uses hand gestures 355 times in the manga according to a 1996 issue of Shonen Sunday. If your wondering what those gestures mean check this site out: JAPANESE BODY LANGUAGE;
Non-Verbal Communication in the Classroom it explains Japanese body language.
The gesture in the above image is called the "oyakusoku pozu" (promise pose). According to a website I read it is a sign of surprise or shock.
Some Random ones:
-Holding up the pinkie is the way to make a discreet reference to having a girlfriend or female lover.
-Batsu is a gesture made by crossing one's arms in the shape of an "X" in front of them, in order to indicate that something is "wrong", "bad", or "no good".
-Two pointer fingers crossing to form an X can also be used to signal for a check.
-Okay in the USA (touching of the index finger with the thumb, forming a rough circle, with the raising of the remaining fingers) means money in Japan.
-Throat slash, aka fingers or thumb across throat in a gesture reminiscent of cutting it, to show one's failure, and could also mean to be dismissed or fired.
-Tugging at the eye, often accompanied by sticking out one's tongue, is a childishly offensive gesture.
-Knocking on your temple with the knuckle of the index finger means " I'm such an idiot".