JSpeed supports rewriting of links for all assets to make them pointing to a custom CDN domain, including CSS, JS files and images.
In the case that links to images result in broken ones, the problem could be related to the relative path that is not properly resolved to the absolute one by the Joomla 'System - SEF' plugin ordered BEFORE the JSpeed plugin.
As a rule of thumb, the JSpeed plugin should always be the last plugin in the order of execution, indeed it's automatically ordered as the last plugin in the list during the installation.
In the case that you have modified the list of plugins and their ordering, ensure at least that JSpeed is executed AFTER the plugin 'System - SEF', otherwise links to images won't be resolved to absolute ones by Joomla when JSpeed processes them for the CDN domain.