Hi, Brooke!
Thanks for visiting the Apple Support Communities. From your post, I understand you are running iOS 12.1.2 on your iPhone 7, and trying to access pictures you have in iCloud. However, you get an error message when you try to download the iCloud Drive app. I would be happy to help!
In iOS 11, the iCloud Drive for iOS app was replaced with the Files app. It sounds like that may be why you are receiving that specific error: Use the Files app on your iPhone, iPad, and iPod touch - Apple Support.
In addition, if you're use iCloud to access and sync your pictures, you might find iCloud Photos beneficial: Set up and use iCloud Photos - Apple Support.
Have a great day!