The oxy_base-uri() Function

The oxy_base-uri() function evaluates the base URL in the context of the current node.

It does not have any arguments and takes into account the xml:base context of the current node. See the XML Base specification for more details.

If you have image references but you want to see in the visual Author editing mode thumbnail images which reside in the same folder:

image[href]{
 content:oxy_url(oxy_base-uri(), oxy_replace(attr(href), '.jpeg', 'Thumbnail.jpeg'));
}