Hi
.avi is a VIDEO format and not an image, and if you check you should see that they are in your Video Files bin and not in the Image Files bin.
As you are referring to them as "Slides" the presumtion is you actually want to insert them as still images.
The quickest way to do this is to go into Storyboard mode. (No matter how short the slip is, it will display as a "slide")
Drag the red cursor line along to the centre of the .avi "Slide" and then click the "Snapshot" button under the preview screen.
This will place a .png image of the ,avi "Slide" in the Image File bin.
Drag this image down to the storyboard track and drop it next to your .avi clip.
Now right click on and delete the .avi clip.
Change the duration of your inserted image to 8 seconds.
Do this for your other .avi clips.
Nat