Hi
You can create a Photos watch face, but not a slideshow watch face.
Follow the instructions here to sync an album of photos (but not a slideshow) to your Apple Watch:
Choose your album and manage storage
Then create a new Photos watch face to have a different photo from the album shown on your Apple Watch each time that you raise your wrist:
When viewing the time on your Apple Watch, press firmly on the screen > swipe from right to left across the screen until you reach the New option > tap to add a new watch face > turn the Digital Crown until Photos is shown > tap to select it.