Please translate meaning pishi-tsu

A Japanese girl I have known for a short time is about to head back home overseas. On the night before the flight she signed off on messenger with this little cartoon panda saluting... I think it says pishi-tsu.
ぴ し つ
I can't find a good translation anywhere.
Is there a native Japanese speaker who can give a correct translation and interpretation of meaning / intent of this term?
Is it like "final goodbye" (long term) ?
or something completely different? Thanks.

Is this? (a stamp of LINE)
http://ameblo.jp/wimzie/entry-11858209151.html

Not ぴしつ. "つ(tsu)" is not larger letter
small letter "ぴしっ/ピシッ！(pishi'!)" means "beating sound" most
but this ぴしっ！ means saluting (no sound) hand action as onomatopoeia.

First link blogger wrote "Yes, sir!/Yes, ma'am!" meaning as "respect" also right
but she just saying "Thanks!","Have a good day/nice trip","See you again!"
and "You're a good friend/teacher(?) for me/us" maybe.
So not only "long good-bye" meaning no worry.

Similar sound "びしっ/ビシッ！(bishi'!)"and more also same meaning
not only using human "hand(s)" actions usual.
http://tangorin.com/general/with%20a%20snap
http://thejadednetwork.com/sfx/index/hi/9