What layering software can i get to put two pictures in a sequence?

I have two pictures of a person. One is a person smiling and the other is not. I would like to combine those two pictures, which i think is called layering and i would like to make it alternate between those pictures to use in email.
I do not have photoshop, but is there any free software out there to help me out.

So to make it simple, in an email there will be the two pictures alternateing back and forth.

Thanks for everyones help and their time for posting replies.

mkrishnan said:

I think you mean you want an animated gif? Perhaps something like Giffun?

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thanks very much, my problem is solved, it works perfect!!