We can capture process instance image using below REST API:
~~~
http://localhost:8080/business-central/rest/runtime/Deployment.ID/process/process.DefId/image/ProcessInstanceID
~~~
This REST API return the process instance image but if the ProcessInstanceID does not exist, it will return the process definition image.BPMS server didnt throw any error.
BPMS server should validate the ProcessInstanceID.