Need black background for the slider images

I would like the white space around slider images to be black.

I don’t know the ideal dimensions for the header slider images, no response to thread. So I just pick large sizes, which doesn’t work out that well. (large images get cut off or centered with less than ideal results – but not a big deal) I have some images that render oddly on Android – you can see white space around them. I prefer the space to be black.