I'd go for 'me', not 'I'.
Going back to definitions, you'd find 'me' is used as an object whereas 'I' is used as a subject.
In your case since it is not a complete sentence it is hard to say whether we're dealing with a subject or object.
But it is likely that what you mean is "(This is a picture of) Samantha and me" with the first part obviously implied and not spoken/written ; in which case what we have here is indeed an object.
Hope all this makes sense ?