This question was constantly entering my mind, and so I decided to do a bit of research and see what other peoples opinions were, or if there was a certain reason.
I found a lecture by David Deutsch on youtube, which is in parts starting from http://www.youtube.com/watch?v=56o2n8sVvM8.

I learnt quite a bit from it, some of it wasn't completely relevant, and so I wrote down everything that was, I learnt about reasons why flowers might be beautiful to humans, and so I am going to make sure my photographs on the tshirts have these: shapes, patterns, bright colours, strong contrast and symmetry. 

The lecture also helped me realise some things that I want in my adverts, such as including the same things in my adverts that are in my photographs, as this would make the adverts relevant to the photographs, which will probably work well.