All images that are uploaded to your account are processed and stored for re-use.
You can access uploaded images by visiting Media Library > Images - this opens the Media Manager.
Once you have opened the image, you can delete it simply by clicking the red 'Delete Image' button below the photo.
NOTE: If an image is being used by the web builder, you won't be able to delete it until it's been removed from use. This is to avoid accidentally deleting an imported image that is live on your website.