Went back through some worlds, wanted to edit some of the images I used, but couldn't find where the original images were (Probably deleted from Downloads, oops). Anyways, images are saved as 'Base64' strings, which can be decoded into images again, easily, just json editing.
Here's the process to take images out of a world (Works on any downloaded world, helpful if you've lost the original images):
1. Download the world somewhere onto your computer.
2. Access the JSON file directly (just open it), helpful to use software like Notepad++ but Notepad or any text editor works the same.
3. In the file, under;
"worldOverview" look for "thumbnail": and copy the quoted text
"locations" look for "backgroundImage": and copy the quoted text
"entities" look for "image": and copy the quoted text
4. Take this data to a site like Base64 Guru, and simply paste it there. Be sure to remove extraneous bits (quotation marks,'"image":', or anything else).
5. Simply download and use.
Additionally if you want to remove images (readability, file size, idk), you can replace the the entire data with 'null'. Don't forget the comma. 