Saleor uses VersatileImageField replacement for Django's ImageField. For performance reasons, in non-debug mode thumbnails are pregenerated by the worker's task, fired after saving the instance. Accessing missing image will result in 404 error.
In debug mode thumbnails are generated on demand.
Generating Products Thumbnails Manually
Create missing thumbnails for all ProductImage instances.
$ python manage.py create_thumbnails
Image renditions are not deleted automatically with the Image instance, so is the main image. More on deleting images can be found in VersatileImageField documentation