Where could I find a guide to take me to the best photo sites at the Great Wall on Oct. 12th?

Yes, Autumn on Mutianyu will be great and it is really a good place to taking photos.
If you want to find a guide to accompany you on the great wall, I help you to find a travel agency they have this kind of service on Mutianyu. You can refer to https://www.travelchinaguide.com/package/beijing/greatwall2.htm to see their arrangement and ask them to make sure whether they can help you.

Dear Steve,
TravelChinaGuide can be a good choice for you. The guide of our company provides the best service, including your demand of takinf the best photoes at Mutianyu Great Wall in August. There are exclusive trip provided for tourists who wanna visit Great Wall. You can refer to the following website to know amore about the Great Wall trip.
https://www.travelchinaguide.com/package/
And if you are interested in it, you can also mail them. The address is
TravelChinaGuide.com|tours

May it help and good luck then !