Can you take pictures of yourself demonstrating the stretch? I often do this, then add a description at the bottom of the picture of how to perform the stretch. I’ve also recorded short videos, 10minutes or less, for clients giving them the same instruction.
There are also on-line programs where you can create workouts that have stock photos: ptonthenet.com and fitnessbuilder.com are two I’ve used in the past.
I’m sure you have done your research online, but here is one site with some good material for your question:
I hope this helps.