The photos app on the Apple Watch, will allow you to use a picture as a watch face. Just open the Photos app and force press the photo you want as the face and press the Create Watch Face. It will then use the picture as the face with a digital clock.
Yup.
Here's the trick, though: sometimes the Watch's Photos app crops the image where you don't want it to crop. Take this example:
This is the only way I can get this pic to display on the AW without any prior editing. Here it is grouped with other images:
What you can't see is that this is taken from a painting of the Rebel base on Yavin IV. To get the guard post in the pic on the watch, I'll need to edit it myself.
What people sometimes discover is, it's sometimes difficult to use their favorite photo as a clearly legible watch face, even with a simple digital display in the corner.