Use Case:
I have existing images in my “Files” related list on my object and would like to merge this image field onto my google doc template. We also want to merge a photo taken with a mobile phone into a RS Documents PDF file? (using a upload Lightning component to Files).

Reply:
Currently with RS Documents, you can merge any image as long as it’s a public URL so depends on how you build the process to capture and save the URL.

Files can now be shared publicly so if you can save the public URL for the file in a field, then you can use that field to merge on template. If you go to the file and click on File Sharing Settings, share with link, copy that link.  Open it. Then right click on the image displayed to get the image URL itself.

Here is an example: 
Link share URL:
Then when you right click on image to copy the image URL, you will get:
Then you can then use this copy URL (second URL) to merge in contentId value and version ID. If your image URL is more than 255 characters, make sure you are using a LONG Text field so it can fit the full URL.

 

Another option could simply be to upload the image to your ‘Documents’ tab and make it externally available per this article.