It's good that you can see the t-shirt designs right away when the first page loads, but overall the site looks a bit dark and uninviting.
Your image thumbnails look a bit pixelated. As if they were resized by HTML (height and width tags) instead of actually resizing the image.
On the "New Products" page the t-shirts all have transparent backgrounds, so they seem to look gray instead of white.
On the individual products page the product description text is VERY tiny. I would add more description there and enlarge the font.
The whole shop says osCommerce in the "TITLE" bar (one of the most important parts of your web page). It should say the name of your shop when you're on the homepage and the name of your products when you're on the product detail page.
You should carry larger sizes than LARGE
It looks like this came from a predefined oscommerce template and not something that was designed for your site and t-shirt theme. Most customers probably won't notice this, but if you can get a site that looks more open and inviting and helps your designs stand out a bit more, I think it would help your sales potential.
The coat hanger and bird icons currently seem a little incongruous, though I guess you may integrate them more later.
There's no point having a choose your language button and a currency dropdown when there is only one option in each.
As Rodney said it does look like a template, and it wouldn't hurt to customise/personalise it a bit - but on the plus side it does look like a good template. It doesn't look like every other OsC site, which is good.
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