The Prewear.com web site is using
PrestaShop Free Open-Source e-Commerce Software for Web 2.0. But, if you don't have experience, this is not what you want to use. If you're looking for an easy system to use, you'll need a do it yourself system that can accomodate people who don't know what they're doing. There are many out there - take a look at my sig for one.
Regarding superimposing your images on clothing. This is something you can easily do yourself. You do not need high resolution images for the web - this isn't print. You only need high res for the web if you're expecting people to be able to zoom in on designs - which you shouldn't because those kinds of features cost a lot more money and superimposing is going to be a lot more work.
For the web, a good picture size in about 500x500. The one image I checked in prewear.com is 600x600. This means that you just need to find plain pictures of shirts that are within that range - you can go as far down as 300x300 and just do a "zoom" on your design. You can pictures of plain shirts pretty much anywhere but you should start with the company from whom you actually buy shirts. Then use PhotoShop or whatever graphics program you use and superimpose the images. If you need more specifics on how to do this, feel free to PM me.
You can hire someone to do this for you but I am not aware of any prepackaged program that will do this for you. Just because I don't know about it though, doesn't mean it doesn't exist.
I hope this answers some of your questions.