cannot find how to add attribute to <img src>

Hello,

I am trying to find out how I can add an attribute to my images on my blog. ForestWander Nature Photography

I need to be able to add a tag to <img> for a java script pop up to allow people to link to my images.

I am using this script
https://www.funscripts.net/javascript-widgets/
and cannot find where I would add the variable to the <img> tag.

Thanks for your help…