Incidentally, if you're entertaining any idea of selling your designs and you're planning to use someone's photo (as in your first design above), you'll need to get permission from the photographer, otherwise you are breaking copyright laws.
This place has some interesting designs:
http://www.redbubble.com/search/bicycle/clothing
I don't like all of them, and wouldn't want them on a boxy T-shirt, but some are nice.
This one is kind of cute:
http://www.redbubble.com/people/lizz...bike-australia
This one would be pretty, especially if the image were larger and the T-shirt were in a purple or light blue or something:
http://www.redbubble.com/people/dixo...n-garden-shirt
I like the way the image is off centre on this one, and it would be a tough choice to pick a colour:
http://www.redbubble.com/people/zann...5-2-retro-bike
These places have a whole bunch of cycling T-shirts as well:
http://shop.cafepress.com.au/bicycle
http://www.zazzle.com.au/cycling+tshirts
This is nice in a navy blue:
http://www.zazzle.com.au/cycling_fem...87593636609106