With 20 images I sequence the layers. The sequence plays out for four minutes.I adjust the Composition setting, duration ; and in Animation, sequence layers, duration. I keep making adjustments and end up with at a best four minute timeline. I cannot figure out how to adjust every photo so that in sequence they fit into the audio track.
I always render so that I can import into Premiere Pro to see how the slideshow fits into the audio track. The audio track is 10 minutes. How do I increase the 20 layers of images so that the slideshow can play out for the duration of the 10 minute audio track?
I don't understand what you are saying. If you need a comp that's 10 minutes long you set that in the Composition Settings. If you want the composition to match the length of your audio track then either set the composition to the same length in the composition settings or drag the audio track into the new composition icon at the bottom of the Project Panel.
If you trim the photo layers to the length you need and set their in and out points they will match the audio.
If you don't know how to do any of this then please go through the Getting Started with AE materials provided as a link on the main page of this forum.
Check your import preferences for still duration. Seems like you have set it to a fixed length and should rather set it to comp duration and thus you can't trim them freely. Still, since they are merely stills, you can always use Layer --> Time --> Time Stretch to extend them with no impact on quality.