Watch app shows I have 15 photos on my watch

iPhone, Watch app, My Watch tab, Photos, is Photo Syncing turned off?

If it is then I suggest you unpair and set it up again from the iphone.

If necessary set it up as "new"

...

First unpair your old watch to remove data and activation lock 

 Unpair and erase your Apple Watch – Apple Support

(this will create a backup too) 

Then set up your new watch 

Set up your Apple Watch – Apple Support

which includes this article how to set up cellular 

Set up cellular on Apple Watch - Apple Support

or in UK 

Set up mobile on Apple Watch – Apple Support (UK) - takes about 20 minutes

I had the same issue and was able to fix it.

It appears that the Watch synced some photos that it shouldn't sync. The fix was to delete these photos on my iPhone.

To identify them I did the following:

1. Sync an album with one picture
2. Create a picture watch face using the previously selected album
3. The watch would then not only show me the one picture from the selected album but different random pictures which I then searched on my phone and deleted from there

It was a bit tedious to find the right pictures on my phone but after then removing that album with one picture from the sync again the watch app on my phone shows 0 photos synced.

This phenomenon has just appeared on my watch and I thought of Watch screen shots. So I took another screenshot and the photos count incremented by 1. You could see if your count goes up by 1 if you do the same.

AFAIK the photos go direct to iPhone and cannot be viewed on the watch, but I don't know if they take up measurable storage. Removing them would be the same remedy as I suggested above, I cannot be bothered either!