"My grandmother posted a photo of her dog!"
Translation:Ma grand-mère a posté une photo de son chien !
I don't know, the English sentence is about a dog in general without precising its gender, which would be "chien" in French.
Female dog = chienne
Male dog = chien
Dog = chien
"I like dogs" would be "J'aime les chiens". The person talking in the English sentence is not precising if it's a male or a female after all.
It seems it's because "grand" in "grand-mère" comes from a time we didn't have feminine forms for adjectives, and this expression stayed like that.
It seems there is a debate about agreeing "grand" with plural of "grand-mère". Different dictionnaries don't say the same thing.
You don't agree the past participle with the subject if the auxiliary is "avoir", and you agree it with the direct object if this direct object is before the auxiliary.
Ma mère a posté une lettre. ("ma mère" is subject with "avoir" auxiliary)
Ma mère est tombée dans les escaliers. ("ma mère" is subject with "être" auxiliary)
Les lettres que ma mère a postées. ("ma mère" is subject and the auxiliary is "avoir" but the direct object "les lettres" is before the verb).
Les enfants à qui ma mère a parlé. (in this case the object is before the verb, but it's an indirect object, so no agreement with it)
Good f- luck with that my friend, many French people struggle with it. And I've not mentioned everything.