Right-click on the layer in the Layers View, and select Duplicate Layer. Create as many duplicates of this layer as there are phrases in your spiel, then go in and edit the text in each one. Of course, you could also have selected the text and copied it, then pasted, which automatically creates a new layer. Using Duplicate Layer, however, the new layer is perfectly lined up with the old, without having to drag the text into place. Whatever you do, make sure that the text on every layer lines up with the rest vertically and horizontally.
Nothing looks more amateurish than text that shifts position slightly as it changes.
Now that we have each phrase of our message on its own layer, all we need to do make each one visible in turn, and export to a unique file. Now export this file as a .gif, then turn on only the second layer, export that as a .gif, and so on. We now have several .gif files, each of which represents one frame of our animation. Open the animation shareware package of your choice, and use it to assemble these files into one animated .gif file. We used text in this example, but the same technique can be used with pictures just as easily.