put some effort in and you will be able to design stuff like that, of course you will need software - photoshop and illustrator if you are a pro you should have these. I would recommend that you sign up over at deviantArt where you will be able to grab resources for doing grunge design, gomedia is ok but no comparison with the sheer amount of FREE resources at deviantArt.
here are a few links directly to the resources:
deviantArt.com - Home
dA application resources - Brushes, Shapes, Actions and so on for Ps, Ai, GIMP and co.
dA resources - General Resources for Designers and (Digital) Artists: Fonts, Stock Images, Textures, 3d Models and so on.
dA Tutorials - Learning by Doing is the way to go, Photoshop techniques, Drawing, HTML, 3d Modelling - Get your Knowledge.
the Deviants themselves offer some wicked stuff and are the Real Resource - visit a deviants page - check out the deviants
Gallery, if you like their work then the chances are that you will find stuff in their
Faves that is relevant to the type fo work they are doing (and that you want to do) - click on a Fave and you will be whisked off to the next Deviation or Deviant, finding resources this way is much more fun and inspiring than just cruising through brushes and shapes preview thumbnails.
dA Search - with the Deviations submissions counter at 65,380,023 !!! WOW over 65 million submissions as I write this (check the score now
HERE) you can either surf dA for the rest of your days and not have to see the same thing twice, or effectively find the kind of stuff youre looking for by using the
Search function which works both sitewide or contextually, with the massive amounts of data that gets pushed around at dA thats no small feat.
Once you have started to REALLY TRY to design stuff like this you won´t be able to stop. And dA will probably become your second home...
Of course getting that "Great Design" down is not even half of the battle - How to get that cool stuff you did onto t-shirts is the matter at hand here at T-Shirt Forums, and the Technical Help is why I am here in the first place, I can design the stuff but now I need to start getting it on chests and backs and sleeves...