I did what many graphic designers have probably done at one point or another: search the Internet for images and recontextualize them into something completely different.
I wanted to a nature picture that demonstrated characteristics of Fibonacci Sequences and the Golden Mean, and I found this amazing page by Dr Ron Knott with lots of great pictures and information.
I took his picture of Romanesque, and remixed it into my banner above. I'm going to contact him and see if it is okay to use it on my site.