Hello guischarf,
Thank you so much for using our theme SuperMag. As per the instructions mentioned on the doc, if you use the acme fix image, it will help to make the image size as our demo. Another things is that, you have options to change the image size on the widget setting as well. Please check that options too.
P.S. If you add the high quality image, it will be far better to make the site awesome.
Regards
Acme Supports