If some parent project of the project which contains the processe diagramms is closed (E.g. if you have an Maven Project with an Parent Project which you don't need to have open), no PNG gets generated.
Instead of this, you will get an entry in the eclipse error view which says the following: "Unhandled event loop exception during blocked modal context."

org.eclipse.swt.SWTException: Failed to execute runnable (java.lang.RuntimeException: java.lang.reflect.InvocationTargetException)
at org.eclipse.swt.SWT.error(SWT.java:4397)
at org.eclipse.swt.SWT.error(SWT.java:4312)
at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:138)
…...............
at org.eclipse.jface.dialogs.ProgressMonitorDialog.run(ProgressMonitorDialog.java:507)
at org.camunda.bpm.modeler.runtime.engine.util.DiagramExport$1.run(DiagramExport.java:271)
... 84 more
Caused by: org.eclipse.core.runtime.CoreException: Can not save image as file: Resource '/wasabi-activiti/wasabi-activiti-freenetmailorder/src/main/resources/processes' does not exist.
at org.camunda.bpm.modeler.runtime.engine.util.DiagramExport$2.execute(DiagramExport.java:317)
at org.eclipse.ui.actions.WorkspaceModifyOperation$1.run(WorkspaceModifyOperation.java:106)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2345)
at org.eclipse.ui.actions.WorkspaceModifyOperation.run(WorkspaceModifyOperation.java:118)
... 88 more
Caused by: org.eclipse.core.internal.resources.ResourceException: Resource '/wasabi-activiti/wasabi-activiti-freenetmailorder/src/main/resources/processes' does not exist.
at org.eclipse.core.internal.resources.Resource.checkExists(Resource.java:341)
at org.eclipse.core.internal.resources.Resource.checkAccessible(Resource.java:215)
at org.eclipse.core.internal.resources.File.create(File.java:114)
at org.camunda.bpm.modeler.runtime.engine.util.DiagramExport$2.execute(DiagramExport.java:307)
... 91 more

This just happens if some of the parent projects (In this case wasabi-activiti or the on over wasabi-activiti) is closed.