As I understand it, and I don't think I'm wrong in this, the Apple system doesn't allow for what you're asking. If you just want to see a photo or photos of your grandkids along with the time, there are some clock apps that also have pics that ch be added. SplashData's clock app is great, just turn on the iPhone to never lock in settings, then crop some photos to size in Photogene, and import them to SplashData's clock app, and you've got both the time and the photos on screen (and you've got a few photos rotating in slideshow, a plus over just one photo).