I am not quite sure what your talking about here teal? Are you talking about a changing picture like on this site? http://www.basinwaterfowl.com
(sorry for the spam, it is the only site I could think of that has the changing flashing picture)
There is two ways, one way is to use an animated gif program to make a gif file that rotates between two files, the other way is to use a java script or similar to rotate through the picture files.
I think your looking for an animted gif program though.
I used a program like that to make this gif file for an avatar on another site.