That t-shirt designers done in flash, try looking through flash resource sites.

You'd need a combination of flash, php, databases so i'd imagine

Possibly, you could avoid flash, with some of possibilities of ajax...

However you have to weigh it up... flash requires a plugin, ajax doesn't work on older browsers....

Ajax is better for SEO flash can't be read by search engines.... theres so much to consider.

Flash would be the simplest and most probably the best method imho.

I don't know much about flash or actionscript tho... try googling dome resources.