We received a christmas photo from my husband's mother. It included his mother, his siblings, their kids, and his ex wife. Is this appropriate?

For the sock drawer, not your wall.
I think she could have used better judgment in a gift.

If she didn't have a photo available without the ex wife in it, she shouldn't have sent it as a Christmas card. She may have been too busy with the Christmas rush to notice? Someone else may have selected the photo and slipped it past her?

Don't lose sleep over it. Laugh and put it into your sock drawer. If she did it to be mean, don't even react. Just watch and see if such a mistake is intentionally repeated.