Slider image size wp explorer minimal xpert

Hi there,
Does anyone have any experience in using minimal xpert wp explorer theme?
I am trying to add some new images to my site as the ones on there now are huge and are causing my site to slow down. When I add an image it doesn’t seem to fit the slide and leaves a gap at the side exposing the previous image. No matter what size I change it to this happens.
You can see here on the main page slider under the header.
http://www.englandculturalhomestay.com

Any help would be brilliant.
Cheers a lot.