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"This is my son and I in Mexico." or

"This is my son and me in Mexico."
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In terms of "correctness", the form "This is my son and I in Mexico" is the right one. This sentence can be an answer to the question "WHO is that in the picture?", and the who-question should be replied with subject pronouns. Object pronouns are related to whom-questions.

However, this is considered a formal structure nowadays. Suject-forms are losing ground in favor of the object-forms in informal settings and this is spreading. "It's me" is more common than "It is I", and some grammarians say it sounds "more natural". Compare:

Does anyone want any more beer? - I do!
vs
Does anyone want some more beer? - Me!

Hope this helps! Emotion: smile
Comments  
Raul's reply was better, so I have removed mine to reduce confusion.
 Raul's reply was promoted to an answer.