Almost 100% sure dynalist is using AmazonS3 to host image assets. Dynalist is pulling the URL for that image asset when you see it in your notes.
Also, you don't really want image data in a raw data format mixed in with dynalist backups. It would really convolute all the backup data and get messy.
The only way I know of right now to download image assets is to just go into your upload manager, find the file URL, download the image from there.
Or, read the OPML backup (found in dropbox/apps/dynalist) file and use a python script / regex to scrape all the URLS starting with
https://dynalist.io/u/ then use something like extremePictureFinder to download all the links all at once
Correct me if I'm wrong @erica ?
also this is why I use imgur and shareX to host all my images