The Messenger of Allah (peace and blessings be upon him) said, "If good is done to someone and then they say "Jazak Allahu khayran" to the one who did the good, they have indeed praised them well." [Tirmidhi]
it is JazakAllah KHAIR not JazakAllah
this is a general information for some brothers & sisters who say "JazakAllah" when someone does something good for them or in general...Source
saying only "JazakAllah" means "may Allah award you" and not "may Allah REward you", so it could be anything...Allay may award some1 punishment also...
"JazakAllah Khair" means "may Allah reward you", which is the right thing to say..
just an information i wanted 2 share...