Well you can start with a basic path and then edit the path or you can create a custom path by drawing it with the mouse.
To do this click the animations tab and select your first image
Click the Add Animation button or the downward arrow with line above it at the end of animations box.
Click any of the Motion Paths or click the More Motion Paths at the bottom.
To customise the path either right click and edit points or click the Effects Options button then edit points.
You can also reverse the path.
EDIT: Then do the same for your second image.
You can also add any other entrance, exit or emphasis animations to the image, eg. a slow fade as it follows your path.