The Sequence Layers command is useful for creating a slideshow from a series of stills.
Select all of the stills you'd like to sequence.
Drag the stills on the new comp icon in the bottom of the project window.
Choose single composition.
Specify which dimensions if the files differ .
Select the duration for the stills here.
Decide if you want to sequence the layers and if they should overlap. Use the Dissolve Front Layer option if you'd like to fade between shots.