The Retailer - Category images blurry and distorted


I am using The Retiler ( and I am very, very happy with it. Thank you.

…But I am spending a hard tome trying fo find the right dimensions for the category thumbs. I tried many combinations and the images are always blurred. Finally I found a solution: he images are sharp but they are off centre and not prepared for mobile devices (the text appears in front of the image)

I also tried to adjust the miniatures in media and regenerate thumbnails but I am afraid this only affects to product images, isn´t it?

Any help will be really appreciated. Thanks in advance.