First you need to go to artes visuales and then the secritaria de cultura to get permission to export any artworks. There are forms to complete.
You need two photos of each painting or drawing.
You also may need an appraisal from the banco ciudad (and another photo).
Or you can just pay a despanchante aduana to do it all (but it isn't required by law).
All of the above applies to all works of art, even a doodle of a flower on a napkin that your four year old made.
If you think the last sentence is absurd, just go to FedEx and ask. That's where I heard that example.
They will not (knowingly) accept artworks for shipment abroad. As the sender you can also "get into trouble" for trying to do so. Laminas (posters/copies) are the exception unless they are individually signed and numbered.
The cheapest way is to do it yourself and take it with you in your luggage.
If you want to do the latter (legally) you will also have to go to the airport the day before your flight and submit the papers (including receipts of purchase). On the day of your flight you will have to go back to the aduana at the airport to have them inspect your luggage prior to check in.