The WPDM Image Button add-on allows you to upload custom image buttons, but it wrongly uploads them into the plugin folder itself so that when the plugin is updated the custom image is deleted and my download buttons disappeared from the site.
Custom uploads should be places somewhere in the wp-content/uploads/ folder so they are not lost. Please update the plugin to place them properly. Also would be great if the next plugin update has code to move the existing download icon to the new location else the download button will disappear again.