Resize categories image with browser window for responsive theme

[gianlu.it]

Using prestashop 1.6, black hawk free template.

Hello, I make images for my categories, 870x217 image size.

I upload for categories image, when I display with fullscreen browser image is perfect, when resizing or using tablet or mobile phone, the image is resized but there is a grey background stripe.

You could see on the attachment what happen.

With default theme that doesn't happen, image is resized correctly.

Thanks for your help!

Gianluca