Plugin Not Fixing Insecure Content

Hi,
I recently installed your plugin on this site: http://contentgarden.org. Using, https://www.whynopadlock.com/, however, it says that there is still one insecure item that’s getting replicated:

Total number of items: 66
Number of insecure items: 1
Insecure URL: http://contentgarden.org/wp-content/uploads/2015/12/IMG_6598_B_updated_2-e1450105389174.jpg
Found in: https://contentgarden.org/wp-content/uploads/themify-css/themify-builder-1121-generated.css?ver=16.11.23.14.59.09

Please advise