Hi,
That's because of three things:
- you've turned on the "force size" setting of the HikaShop configuration so that your thumbnails all have the same dimensions.
- you've uploaded images with different height/width ratio than the thumbnails size you configured in HikaShop
- you've set the "Image scale mode " setting to "keep ratio"
Because of all that, the thumbnails generated have to have that white background.
To avoid that, either use images with the same ratio as you want for your thumbnails, or turn off the "force size" setting so that the size of the thumbnails can be different, or change the image scale mode to "crop" so that the system fits the image to the size of the thumbnails by cropping the pieces to fit the ratio instead of adding white stripes.