If people came to my house and engaged in conversation with me even though I didn't wish to and then took photos of my property outside for unknown reasons, can I sue them for trespassing, harassment and invasion of privacy? If it happens again, is it considered as stalking and what can I do?
One course of action would be to hire an attorney to write them a formal letter asking that they cease and desist. These are relatively easy to draft, and serve the purpose of putting them on notice that they need to leave you alone. This sets the case up in case they continue these actions.
Your limited facts do not reveal any legal damages. Conversations you don't want are simple: don't speak. Taking photos of a residence from the street is normally permitted. Since you don't indicate the nature of the encounter, one cannot analyse this any further.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.