No such file error using Jekyll plugin

I am having issues using the following Jekyll plugin:
https://github.com/mrdanadams/jekyll-thumbnailer

First off I edited line 39 from:
source_path = "./source#{source}"
to:
source_path = "#{source}"

Then when I build I get the following error:
#jekyll build ... Generating... Thumbnailing static/images/sidebar/moon.jpg to static/images/sidebar/moon_50x50.jpg (50x50<) Liquid Exception: No such file or directory @ rb_sysopen - ./sourcestatic/images/sidebar/moon_50x50.jpg in index.html jekyll 3.4.0 | Error: No such file or directory @ rb_sysopen - ./sourcestatic/images/sidebar/moon_50x50.jpg

Where is the “./source” coming from on the error line?

Without editing line 39 I get:
#jekyll build ... Generating... Liquid Exception: static/images/sidebar/moon.jpg is not readable in index.html jekyll 3.4.0 | Error: static/images/sidebar/moon.jpg is not readable

Which makes me assume I am on the right path.