It depends on the site, so you should give those user agreements a careful read. If it's ownership in particular you're worried about, do a search within the agreement for "copyright", there aren't likely to be more than a few sections featuring the word.
In the vast majority of cases, you still own the art. The only thing the company gets is a licence to reproduce the art on products to sell, since that's kind of the point xP Most also stipulate a licence to use your art for promotional purposes, this covers both the ability to present your products in their storefront, and to use them for advertising the site itself (usually on the site, e.g. advertising holiday-themed products).
The other thing to be mindful of is when your licence of your art to the company ends. Make sure you check the termination clause, it's usually towards the end (or search for "termination"). Ideally, it goes like this: When you remove your products from the site, the agreement is terminated. The company owes you nothing more, but also loses the licence to your art. In some sneaky cases, the agreement is written so that the licence is perpetual, so be on the lookout for that. What's nasty about that is that this perpetuity isn't mentioned in the termination clause, but somewhere higher, where it describes the initial licence. So, do a search for "perpetual" and "perpetuum" just in case.