I was curious what would be the approriate board to post this on? I contacted the provider whose photos were stolen, but wanted to alert others who might unknowingly contact her.

I realized that fake photos are pretty much the standard in this industry especially at the mid-low tier. However, your concern is to the point.

We have to do something!

If it was on an ad site like Backpage, stolen pics are almost the rule rather than the exception. If you want, you could post about it on a Regional Board or another local board where hobbyists exchange tips and warnings.