Absolute paths (starting with a slash) should be written relative to the source directory. Let's say you're working on a page at /source/about/team.ejs. If you use <%- srcset('/images/header.jpg'); %> it will look for the image file /source/images/header.jpg.
Relative pths should be written relative to the current file. Again, let's say you're working on a page at /source/about/team.ejs. It you use <%- srcset('images/header.jpg'); %> it will look for the image file at /source/about/images/header.jpg.