If you create your site on one server and then move it to another server, the images will likely all disappear. This is because Smart Slider 2 uses absolute paths to the images rather than relative paths.
I was successful using the first method below.
http://www.nextendweb.com/wiki/smart-slider-documentation/frequently-asked-questions/#localhost
After you make the changes, you may need to refresh the cache. Go to Smart Slider 2 and choose Settings then select Cache. Click Refresh Slider.
http://www.nextendweb.com/wiki/smart-slider-documentation/global-settings/#image_url