This is a limitation of the current Previewer. The issue is that relative resource paths are not known to the browser component. The challenge is that the browser component will only load content from a file. Also we wanted users to be able to preview source that is dirty (not saved yet). Presently when preview is invoked the editor saves its working content to a temp file outside of the containing folder. Thus relative resources are not located by the previewing browser component.
We have a work around that we will make available in an upcoming release.
Michael
MyEclipse Support