How big images should I use?
If you are using a full width slider, then the most optimized would be, if you would use images with 1920px width, and the height should be, as much it has to be to have the same ratio, what your slider has. So for example if you have a slider with 960px*150px size, then your images should be 1920px*300px.
But if you are using a smaller slider, then you should use a smaller image. So the key is, that you should have as big images, as big maximally your slider can be. The 1920px width is suggested, when there is no limitation, because that is the "full HD". Most people, who will check your website will see the slider at that width or in a smaller one.
But if you are not worried about having optimized images, you can use images with any size; our system will strech them correctly to be in the slider, and the Default slide background fill mode option will decide, how they will be inside the slides.