Is this plugin affecting website SEO?

It should indirectly effect your SEO positively due to faster page loads and fewer visitors clicking back to the SERPs (which Google does indeed monitor).

I have not registered any negative SEO effects by using this plugin. I have also worked with some industry leading SEO consultants who haven’t seen any negative effect either.

The more advanced search engines (like Google) have started picking up content inserted by JS, and the lesser advanced still use the noscript-elements where the original image elements are displayed.

I have just a few images on my personal site, but they are all indexed by Google:
https://www.google.no/search?q=site:bjornjohansen.no&tbm=isch