Image processing is an important topic in which clustering has some application. It's worth pointing out that there are several very good image processing libraries in Python. scikit-image is a sister project of scikit-learn. It's worth taking a look at if you want to do anything complicated.
A big point of this chapter is that images are data as well and clustering can be used to try to guess where some objects in an image are. Clustering can be part of an image processing pipeline.