How to use Rules' "set data value" with image fields?
I do something similar on my site; though the solution will vary according to where you are getting the image file from.

On my site, I first use a "Download a file from the internet" action to fetch and save the image file I want.  When you are setting up this action, be sure to scroll all the way to the last fieldSet on the page - Provided variables - and make a note of the machine name there(or change it to something more memorable) 

Next add a "Set a data value" action, and use the machine name from above as the Value.

[edit] I have just realised - while looking down my module list - that the "Download a file from the internet" Rules action is provided by a module, here is a link:- https://www.drupal.org/project/rules_download