[JWait]

By default, X-cart will display the thumbnail when you don't have a product image. If you have a product image it will display that instead of the thumbnail. If you have variant images defined then it will display those according to the choice made in the options selection box. If you have detail images, those are either opened in a popup window, or displayed below the product details, depending on the choice made in admin/general options/appearance options about having the images in a popup.

am2003 - "I defined an option named color (Red, Blue, Green), and added an image for every color." Did you define a image for each variant? If you delete them the product image (if you have one), or the product thumbnail will be the only thing displayed.

Gijs - Have you made any modifications to your installation of x-cart? What you describe is not normal behavior. The product thumbnail should not be loading if you have a product image defined. I am thinking you may have a 3rd party image mod of some sort that is causing this, correct?