When we user larger images in the header for example the header is loaded white or grey depending of background color set. Then the image is loaded.
If it is a slider the images are loaded when the slider changes the slider not before.
You need to pre load images or give us a setting checkbox if one image should be preloaded or not. This would stop image flickering when you have one image and loads another on top of that.
I also want to use preloader for stuff outside viewport so it do not render before someone starts scrolling. This would speed up things a lot.