Thank's for help.
I can use the rest service of gwt-console-server to get the process image: http://{your-address}/gwt-console-server/rs/process/definition/{your-process-id}/image.
But I don't need the gwt-console-server because it use the service of guvnor: http://{your-address}/drools-guvnor/org.drools.guvnor.Guvnor/package/{your-package}/LATEST/{your-process-id}-image.png.
I can also access the image with following rest service of guvnor: http://{your-address}/drools-guvnor/rest/packages/{your-package}/assets/{your-process-id}-image/binary.
The main problem is that the process image will not be generated. You must create the image manually in designer first (with PNG button on bottom). After creation guvnor put's the image in the same package as other (PNG) asset.
So if you forget to create the image on every update of the process, the image is not available or not up-to-date.
