How to load rootspersona images over https

Getting a warning:

Mixed Content: The page at ‘<URL>’ was loaded over HTTPS, but requested an insecure image ‘<URL>’. This content should also be served over HTTPS.

I believe that the image referred to may be:

https://www.mydomain.com/wp-content/plugins/rootspersona//images/boy-silhouette.gif

Is there any way to cause this image to load via https? Perhaps editing the database using phpmyadmin?