Best of luck. I am a dog person so hope you do well. I would suggest sharing some concrete information if you have documentation of being a true non for profit.
As much as those pictures pull at heart-strings and Facebook does, that is not concrete proof. I would provide it in open not just PM as you mention. It's just a hard sector as so many scammers out there.
Yes those photo not a proof, we can just search it online. If you can take picture with your name and yourself it will make people more trust