Create an object store (Amazon/Bluemix). Fetch the images from URL via the storeURLToFileDocument action.
Use the SFTP module from the appstore and use this to transfer the images. We use this module all the time to sync photo's and documents from clients to our environments.
Width the dropzone widget you can drag and drop a folder of images. 3000 might be a bit too much though.
An other alternative would be to create a java action that could unfold a zipfile with your 3000 images and convert them to a Mendix image entity.