I have a question about articles in so-called 'of'-phrases. According to this thread, nouns followed by a modifier are usually accompanied by the definite article (if only they aren't concrete mass nouns and plural nouns). Yet, I frequently see that some phrases can also be accompanied by the indefinite article a/an, like for instance in the example from this guide:
This is a picture of my family.
I got a discount of 10 percent on the purchase.
It seems to me that usual rules that guide the usage of the articles do not always work here. Can anyone give me any advice on how to choose an article in such situations, please?