Hi lucindapoppy, 

I understand you installed a wallpaper app on your iPhone SE, and it has changed your photos.  The easiest way to get them back to the way they were before, is to delete the app from your iPhone.  The iPhone should revert back to whatever it looked like before you added the app.  You can always redownload the app if you want to from the iTunes Store.   Here's how to delete the app:

Delete music, movies, apps, and other content from your iPhone, iPad, or iPod touch

Delete an app

1. Press and hold the app until it jiggles.
2. Tap the  in the upper-left corner to delete.

You might be asked to confirm that you want to delete the app. If you see a pop-up that says, "Deleting [app name] will also delete all of its data," tap Delete.

 

 

lucindapoppy, if for some reason this does not resolve your issue, you can reset your iPhone from the Settings app:

iPhone User Guide - Reset iPhone

Reset iPhone

Go to Settings > General > Reset, then choose an option:

• Reset All Settings: All your preferences and settings are reset.

• Reset Home Screen Layout: Returns the built-in apps to their original layout on the Home screen.

As a last resort, if you have a backup of your iPhone, you could also restore your iPhone to the date just before you added this app.  

Restore your device from an iCloud or iTunes backup

Hope this information helps ...