I'm trying to explain why the two examples below use the definite article and no article specifically.

A garden gnome was stolen and taken around the world. The owner of the garden gnome received a letter with a photo of the gnome in France. 1) THE letters continued every day for a month, with 2) (no article) photographs of the gnome in different parts of the world.

Can we alternate 1) or 2) with either the definite article or no article? I understand we can use the definite article to refer to introduced nouns and for specific plural nouns within a greater group. But I'd like to understand how the no article in 1) and the definite article in 2) would work here. Thank you.