Writing is a struggle against silence. ~Carlos Fuentes

All you have to do, once you have your animation window open (it's the long rectangular one) is open a few new frames, then go to the file drop menu File, then scroll down to Place and pick the next picture you want to insert, then repeat.

Then go to the layers pane and click on the box with the eye in it- if you don't want a picture to show up in the sequence beyond it's first appearance, you click on the eye once and it goes away and then that frame is free for something else.