If, during processing, Symbiostock discovers that a media product you have is missing its corresponding original file, it will move the status of that file from public/draft to pending. This means you should consider replacing that image by uploading a new copy.
There are very few reasons this will happen, but this acts as a fail-safe to organize all the problematic images in case this situation should arise. Also note that Symbiostock does not naturally check to see if the images for the products are there – it assumes they are, but if for example, during thumbnail regeneration, finds that one is missing, this fail-safe gets triggered.