I covered the boards with an advert from a magazine. If the cover looks familiar, you obviously live in a country that has Target stores.
Since I am not selling this book but will be using it for myself, I don't think Target is going to go after me for violating their copyright, but if you are going to make books like these and sell them or even give them away, you had better make sure that the stuff you use on the cover is
a) in the public domain (i.e. copyright free),
b) under a creative commons licence that allows this kind of commercial use,
c) you can obtain a licence from the copyright holder, or
d) you hold the copyright.
I am working on using my own photographs to cover books to be sold, but using photographs or photos printed at home on photopaper is not easy, as PVA glue doesn't hold them. Using regular photocopy paper is a problem too, as laser printed photos on regular paper are rather delicate (I use this for origami and the paper shows through in every fold) and the photos I have printed with my inkjet printer tend to smear. I will probably have to invest in some special paper and ink if I want to do this, or find a suitable glue that will bond photographs to bookboard.