The only place where the icon is to activate the slideshow (aka: make the iPad a picture frame) is from the Lockscreen. So, put your iPad to sleep (press the Sleep button once ... and briefly). Then, press it again to reveal the lockscreen. You should see that little flower icon in the bottom right of the iPad (next to the "slide to unlock" button). Press that icon and off you go!
Hope this helps.
Marilyn