In order to be able to export diagrams from Papyrus, you will have to install an external plug-in that you may download here: https://sourceforge.net/project/showfiles.php?group_id=110332

Once, you have downloaded this plug-in, you may read the following instruction for installation: ExportImagePluginInstallation.pdf

Original instruction can also be found here: http://www.se.eecs.uni-kassel.de/~thm/Projects/ImageExport/index.html

Finally, here is a video showing how to use it within Papyrus :