Hi Michele -
The easiest way to set this type of slideshow up is to pair an app designed for this type of use case with Apple's Guided Access.
As full disclosure, I work for a company that makes Kiosk Pro, a kiosk app for iOS - http://www.kioskproapp.com. Our Plus and Enterprise versions include a screensaver feature that allows you display a series of images and/or video files stored locally in a folder on the device and includes settings to disable touch so you can use the app as a digital sign. There are other apps out there that do this as well that you should be able to find in a Google search.
We do recommend using Apple's Guided Access feature to 'lock' the device to a single app. You can find information about setting up Guided Access here: Use Guided Access with iPhone, iPad, and iPod touch - Apple Support
Good luck with your project - if you have further questions, you can always contact us at our support email: support@kioskproapp.com
- Becca