there are two things you can do:
either use .gif images which are bacically animations already in the picture. There are programs that make it easy to make gif files, just search for them on google, but i do prefer the one picture option below
Or
in a file have several images u draw that you want to have them quickly changing between and name them like manwalk01 manwalk 02 and so on, then when importing them in GE select multiple files and open the first one.
You can also put all of those pictures together into one, taking all the animation pictures and putting them each one after another in the order you want, and increase to how many pictures you have in horzontal frames or vertical frames. What that does is basically for example takes a 500x100 picture and if you set 5 vertical pictures it will take
_ _ _ _ _
l_l_l_l_l_l
1.2.3.4.5
and animate pictures 1->5
I hope this is understanding and useful