We are thinking about to offer a second option for "Manage Designs". Right now you need to upload all images to your WP media library and then select the designs from there. Another option would be to manage designs via FTP folder. So instead of uploading the necessary images for designs into the WP media library, you will have a separated directory in wp-content/uploads. In this directory you can create unlimited folders and sub-folders. Every folder represents a design category and you need to upload the images via FTP into these folders.
In the backend you are able to assign the category and single design options.
What do you think?