You can change the length by selecting the end of a clip with the selection tool and extend it left or right. Also you can Right click a clip (photo) in the timeline > Speed/Duration > Change it to your desire time length. Premieres default images time length is 5 seconds, you can change it by going to preference > General > Change Still Image Default Duration to the time you want. (Note: This will affect every image you import)
“…and will tell you that for MOST projects 5 seconds works.”
I agree with Earl, for slideshows 5 seconds works in most situations. It depends on your taste, but remember the longer the slideshow, the more boring it gets. That why slideshows created in Windows Movie Maker tends to be awfully boring. Their creators are usually not pros and they believe that a slideshow is better when it have hundreds of pictures with transitions on everything (God, I hate that). A good slideshow is one that tell a story with a proper and meaningful order.
