There is a quite old issue on github that I could confirm. As the post didn't get any answer, I'm posting the issue here as it's the official forum referenced on the plugin page (maybe the github issue queue is not considered).
Here is the original message :
the plugin is working great on every other file but not on files from media gallery? Why is that?
See the original post here : https://github.com/OFFLINE-GmbH/oc-responsive-images-plugin/issues/16