you need to have a keyframe on each area in the timeline that you wish to have a new image and the best way is to go to file>import>import to library, then click on the image and in your proporties box you will be able to click Swap.. then select the next image.

or

even if you want to put them in all at once go to file>import>import to stage and they should all go in in there own key frame

Good Luck