If you have recently updated to Enfold 4.0 or higher and some of the images of your default layerslider are missing: this is because you were using some of the default images that were shipped with the theme, but at this point are no longer part of the theme folder, due to a change of how the Layerslider works. If thats the case you should see this image which redirected you here:
However, you can import those images manually with the new layerslider. First of all you need to download the zip file with the slider images
Once that is done, simply use the layerslider import tool to import the new version of those default sliders. The images will be added to your media library. You can now simply replace the missing image urls with the images from your media library.