Welcome to Apple Support Communities.
If I'm understanding correctly, you are having some trouble with 3D Touch. This is a great feature, and I will be glad to assist.
Check out the steps in the following article to see if it helps: Use 3D Touch with VoiceOver and Zoom
Additionally, try these steps to choose a Live Photo as a background from our article: Change your iPhone wallpaper
Choose an image
Choose an image from Dynamic, Stills, Live, or your photos. The default wallpapers change with each iOS update.
To choose a Live wallpaper or a Live Photo, you need an iPhone 6s or later.