I'm afraid that is not possible to retrieve the photos from your Apple Watch.
Photos can only be synced to Apple Watch from the paired iPhone. They cannot be synced in the other direction or otherwise exported from your Apple Watch.
Even if this had been possible, the photos that are stored on your Apple Watch have been automatically resized to smaller pixel dimensions, for display on the smaller screen.
The best that you could do is to take photos (using a replacement iPhone or other camera) of the photos on your Apple Watch.
When you are ready to do so, before being able to pair it to a replacement iPhone, you will first need to erase your Apple Watch manually:
- On your Apple Watch: when viewing the time, go to the Home screen (via a single press of the Digital Crown) > Settings (cog icon) > General > Reset > Erase all Content and Settings.
- Then pair it by following the standard process:
- Set up your Apple Watch - Apple Support
Credit for this answer to Jonathan UK.