I concur with Jennifer S.
I light of crimes against children and privacy issues nowadays, I myself as a Mom, explicitly explain to my Mom/sister/relatives, that ONLY if I "approve" may they use my kids photos anywhere, be it online or in hard copy like in a card. Schools are the same way...
That being said. It is up to YOU on "how" your child's photos are used. AND, how any information about them is dispersed.
For me, it would be irksome, about what your MIL is doing, because she is 'assuming' that she can. Without thinking about your answer or concerns, first.
If there is overlap in your Christmas cards, then so be it.
YOU have every right to have a x-mas card of your child no matter what MIL does. SHE does not supplant your choices. If she grumbles, then so be it. Explain he is your son, and this is your card... and tell her ALL parents do that. It is your freedom to do so.
NEXT, perhaps your MIL can have her "own" card/photo of your son, that is a photo of her WITH your son in the picture, of them together. That can be an option.
YES, it is your turn to show off your precious son in a Christmas card/photo. So do so. You and MIL don't have to use the 'same' photo nor the same card design.
(by the way, I'm all for a MIL that is happy and wants to show off photos of her grandkids too... but that does not mean that the child's Mom ALWAYS has to succumb to a MIL's wishes or be silent about it or give in, all the time. The "Mommy" (& Daddy) has to be primary in making decisions for her children. Ultimately. AND the MIL has to "learn" to take a backseat too, and graciously learn her "place" in the family.
All the best,
Susan