WARNING ! 

If you generate with your current install the path of images will be : platform:/resource/org.eclipse.papyrus.model2doc.integration.gmf.template2structure.tests/output/image/< PATH_TO_THE_FILE >
However in the test environment the path to the generated image is platform:/resource/< TEST_METHOD_NAME >/output/image/< PATH_TO_THE_FILE >

So to make the test works you should replace in the pds file every "platform:/resource/org.eclipse.papyrus.model2doc.integration.gmf.template2structure.tests/" by "platform:/resource/< TEST_METHOD_NAME >/"