How do I use my photo as screen saver
How do I use photo from my iPhone as screen saver to my Apple Watch?
Apple Watch SE, watchOS 7
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There are no screen savers as such for the Apple Watch, but you can use the Photos watch face to show them. When you lower your wrist, a dimmer version will appear, if you have enabled the Always On feature.
Apple Watch faces and their features - Apple Support
While you can do it on the watch directly too, I recommend using the Watch app on the iPhone to set that face up, especially the first time around.