If you define an image in a template, based on a URL and you create HTML from this template, you get a link to a png and the png is saved next to the HTML.
However, some customers want to keep the URL in the HTML, so the image should not be replaced, but should still refer to the URL

There is a configuration option for this on the HTML output format: No Rewrite URI Schemes.

Also available on the output format:
HTMLOutputFormat.setNoRewriteURISchemes(String)

For example:
outputFormat.setNoRewriteURISchemes('http')