AFAIK this is not currently possible. In jBPM Designer you have the option to save the process SVG representation every time you save a process (or when autosave kicks in) - https://github.com/droolsjbpm/jbpm-designer/blob/master/jbpm-designer-client/src/main/resources/org/jbpm/designer/public/profiles/jbpm.xml#L51
with this you can have your svg alongside the process and convert svg->png if needed using batik or some other lib
HTH