pronouns - Is it "me" or "I" and why?

When do I use “I” instead of “me?”  
John, Valencia, and I (or me)?

I found a photo of Sarah, Thomas, James and I?

OR

I found a photo of Sarah, Thomas, James and me?

"I" subject pronoun, photo "object", "me" object pronoun - or not as the case may be?

I believe it is "me" not "I" but can you provide why this is so in terms of sentence structure.

Thank you