I spend a lot of time researching historical themes and I end up collecting a lot of old imagery. I then try to pick out the best images that will translate into cool shirt graphics and that may also have some relevance to current world events/issues... I initially wanted to choose themes from the past that people nowadays might find relatable and maybe even give them insight (learning from the past, that sort of thing) but I'm thinking most people just want cool tees so I'm giving em cool tees.
Most of the imagery I've used to date have British themes but I'm interested in all world history so I'll be branching out in the future.
Below are some images I pulled for Inspiration on the past few rounds of shirt graphics. Click image for larger view.