1) Make sure you have selected a featured image for each post/page/slide.
2) Make sure you have not set the image width to 0 or left it empty on the Theme Options pages, Also make sure you've not set it to 0 (it can be left empty) on individual post/pages and shortcodes.
3) Make sure you have an image editor installed on your server such as GD Library or ImageMagick. If you are not sure please ask your web host about this.
4) CDN hosted images are not supported - the images need to be uploaded to the server the site is on.