So according to this (x),
during Avengers shooting Hiddleston stole the prop hammer and ‘took lots of different pictures’ and naturally 'sent them to Chris'.
He still thinks Hemsworth got them.
"Chemistry is a rare thing, undefinable, that people talk about in movies all the time. Usually it happens between men and women in romantic comedies, but Chris and I have a great chemistry. We love to work together, so it was very good. Thor and Loki are characters that went beyond our expectations in all possible ways." - Tom Hiddleston