How to resize the pics for the slider

Hy everybody It's clear how to set a pic as a featured image. Then the pic will show in the slider, but a section of the picture who is absolutely not logical. how can I Show the whole picture in the slider? As an example you can see the pic with the pyramid. http://rapunzel-will-raus.ch/ It appears perfectly as featured image, but makes no sense in the slider. Who can help me please?