In the second tab "Appearance" change the image dimensions manually (this is more accurate)
NB: Super hint! If you wish an image to resize automatically to the size available on the screen, then remove the height property in the image dimensions, and add 100% (or whatever percentage) to the width property! The image will then scale / resize automatically!
Resize this window and see the difference between these two pictures. One will resize, the other will not: