Fit product image to page width automatically

We’re starting a site that sells banners, most of which have short and wide dimensions (i.e. 8x1, 6x1). The themes we’re using (currently Dawn) cut off either end of the banners in the thumbnail image. How can we have the thumbnail images fit to the width of the product image and display the entire thing?

Any advice would be greatly appreciated!

@Yimble

Please share your store URL!

Thanks!