Got a wedding gift a year after our wedding?

My husband and I got married last July, in a VERY small ceremony (parents and siblings) at the courthouse. Since it was family, and we already had a furnished home, we asked for no gifts. Last week, I got a package adressed to me at my work from Amazon.com. Seeing as I have never shopped online, I panicked and tore it open, thinking someone might have stolen my credit card or something. But it was a very nice cookbook with a card attached congratulating us on our marriage from 2 people I was sure I didn't know, and my husband didn't know anyone by those names either. It took me a few days to track down who it was who sent the gift. It was my parents friends son and his wife (whom I have never met in my entire life). I know I should write a thank you note, but what do I say? I mean, I have no idea why they sent a gift seeing as we absolutely don't know each other, nor am I even sure how they knew where I worked (they live in another state), or even how they knew or cared I got married. What do I say? Should I include a wedding picture? Help on proper thank you note ettiquette?