There is no 'timeout' setting that you can use to say 'after x minutes of inactivity display photos'. So you can either go into the Photos app and start a slideshow from there, or press the sleep button twice (once to lock the iPad and then again to display the unlock screen) and then press the flower icon that appears to the right of the 'slide to unlock' (the flower won't appear if you havn't got any photos) - that will leave the iPad locked but will turn the iPad into a digital photo frame. You can set whether photos are shuffled, and how long they are displayed for, in Settings > Picture Frame (or Settings > Photos for slideshows via the Photos app)