i-slider image dimensions confusion

Hi
There seems to be some confusion over the image dimensions required for the home page i-slider. I’ve seen various posts stating 1920×1080 and 1600×600.
I’ve uploaded image with dimensions 1920×780 to try and accommodate for full HD width screens, but it doesn’t seem to be working correctly.
They are still being resized, and I think its affecting the image quality. I’ve tried entering the following css into the customise > layouts section as advised by another member but it doesn’t seem to affect anything.
.tx-slider img {
max-width: 100%;
width: 100%;
max-height: 780px;
}

What image dimensions/css shall I use for the least amount of scaling/cropping for best image quality?

[ No bumping please. ]