Image url's are always generated in the same manner. If you download an image, you can see which was the original download url. It looks like this:
You can retrieve the guid and application url in a microflow and construct the url like that.
Name parameter is optional but makes sure the resulting image has a correct filename.
Note that you need to have an active session with the Mendix app (even just an Anonymous session), otherwise you will get a 401 error.
You could use the Deeplink module in the Marketplace to create a dedicated URL for the API to call.
The deeplink will need to be a microflow that retrieves the correct image and returns it.
Hope this helps.