there are some good tutorials online
how i do it is open them all in ps, pick one to put the rest on,
then cut out the skier or whatever and a distinguishing piece, like a part of a tree or something
then paste what i copied onto the one you picked to start, this is where the distinguishing piece helps, you turn down the opacity on the pasted image so you can align that distinguishing piece with that same piece on the origional, then turn the opacity back up
rinse and repeat!
like in this one i copied the kayaker, and part of the rock and tree to help line it up