Very simple. Just drag the photo in. trim it to the length you want and add the 'Fade In/Fade Out' behavior found under Basic Motion. Use the HUD (F7) to set the duration of the dissolve.
If your picture's duration isn't long enough, go into the properties tab, and under timing set the end result to hold, or if you find all your pic's don't last long enough change the duration of imported stills in the project preferences.