Right, you can't take, download, copy, etc a photo you find on the internet and repost it anywhere, unless you know for sure that it is in the public domain or you have permission from the Owner of the Photo. People within Ancestry share a lot of stuff but many are not the original owner of the photo/document etc you see on their tree. So if you ask their permission to use it without asking if they are actually the holder of the original, you can find yourself violating copyright rules. This rule also includes documents, copy paste of text from publications etc. What you can do is create a citation listing your Source (in this case a website or newspaper), the website where you found it, the details about the photo/document, include an internet link to the item and include your access date (day you were on the website and saw the item) in your citation.
Hope that clarifies.