We make t-shirts at http://gritandgristle.com
The theme is social protest against people and authorities trying to control our lives and/or tell us what to do. We're looking for something iconic that will define not just the current shirt, but future shirts to come.
Our target audience is rebels, present and former military, anarchists, protesters, etc. Mostly men, but some women so can't be all masculine.
I want the logo to be gritty, grungy, tough, punk rock. Think Harley Davidson attitude, tactical, even military shield/emblem. Think about something that a biker would wear on the back of their jacket.
I have attached some examples, but note that I'm not requiring a skull.
Some imagery that may work:
skull, bicep, guns, muscle fibers, sawdust, bones, ammo casings, eagles, people (tough guy, sexy lady, etc), wilderness, camping/hiking related, etc.