You can lengthen or shorten the playing time of any photo by dragging it longer or shorter on the timeline.
But if you want to pre-set the program so that any stills you add to the timeline will be at a duration other than 5 seconds -- Go to the program's preferences (under the Edit menu on a PC) and, on the General page, change the still photo duration. But you must do this BEFORE YOU IMPORT THE PHOTOS INTO YOUR PROJECT. This new setting will only affect the photos you add to your project after you change the setting. Photos already in your project will remain at the old setting (5 seconds).
On the Timeline, you can vary the length of the photo display by dragging the right side to increase or decrease the amount of time the photo is displayed, as Steve noted. The advantage of this method is twofold. First, it provides a visual clue as to the length of the time that the video is displayed and, second, it allows you to vary the amount of time an image in a slide show is displayed. For example, if you have a set of five pictures taken at different times of day, you may want relatively short shots to give a time-lapse effect, while others, especially ones with lots of different people, you may want to show for a longer time.