This is what it says on Ancestry about searches in private trees:
'Private and searchable
When making a tree private, you may choose to allow people in your tree to be found in searches. Only the names, birth years, birthplaces, and any documents you upload) of deceased people in a private, searchable tree will appear in the search index. Photos will not appear, but limited information about the photo may appear (such as the name of the picture and the ancestor the image is tied to).
If the searching party then tries to view a photograph or other information about the person, they’ll receive a message saying that the image or other detail is in a private tree and that they need to contact that user to view it. The person searching can then contact you anonymously through the Ancestry messaging service, and you can decide whether or not to respond or give them access to your tree.'
This implies that other users should not be able to access photos in a private tree.
Pheno