Featured Images/Slider

I’ve always thought my issue was theme related but it might not be, so if anyone can advise me best on the following that would be great!

I try to add a featured image for my slider using my own images so I upload and set it as the featured image, but instead of the image, I’m greeted by the ghastly red cross on the left hand corner where my slider images should be! I’ve tried reuploading and different pictures, but just certain posts it likes to play up and not display in this manner.

Any thoughts or suggestions? You can view a live example here; http://www.soundsofnowmusic.com

All help REALLY REALLY greatly appreciated thanks

Chris