So your dragHandle.png isn't being found as it's likely in a bundle that's not on the search path. URLs in the CSS files are resolved from the following places:
As an OSGI resource relative to platform:/plugin/org.eclipse.platform/css/ (zoiks! how'd this get hard-coded in there!?)
As an OSGi resource relative to the location specified in the "applicationCSSResources": this property is either specified on the command-line, as a System property, or as a product property. The Eclipse IDE product specifies its applicationCSSResources = platform:/plugin/org.eclipse.platform/images/.
As a URL
As a file path: the current directory usually is that holding the executable (e.g., .../Contents/MacOS/ for OSX)
Plus any resource locators registered with a stylesheet (which should be but doesn't seem to be theme-specific unfortunately).
You have two choices:
Associate a resource locator with a stylesheet. Individual stylesheets can contribute their own URI bases for resolving resources via the "osgiresourcelocator" element on org.eclipse.e4.css.swt.theme/stylesheet. And a stylesheet can be contributed to a theme (see the "themeid" element on org.eclipse.e4.css.swt.theme/stylesheet).
Specify your image using a full URL like platform:/plugin/xxxx/images/dragHandle.png. This is likely the easiest solution.