To download process image through kie-server's REST API it should be exist under default package(/src/main/resources). To move process image under default package we have to clone repository into file system, move process image to defaultpackage(/src/main/resources) and push the repository.
Git commands:
================
git clone ssh://bpmsAdmin@localhost:8001/repository1
git add .
git commit -m "commit message"
git remote add origin ssh://bpmsAdmin@localhost:8001/repository1
git push origin master -f
==============

• create new Process definition under package(com.sample)
• Design simple process definition and save it, process image will be created under com.sample package.
• Build and deploy project, configure container and start kie-container.
• Use below REST API to down process image:
  ~~~
  curl -o ProcessImage.svg -H 'Content-type:application/svg' -u 'bpmsAdmin:admin12@' 'http://localhost:8080/kie-server/services/rest/server/containers/ {ContainerID}

  /images/processes/

  {ProcessDefinitionID}

  '
  ~~~
  Example:
  ===========
  curl -o ProcessImage.svg -H 'Content-type:application/svg' -u 'bpmsAdmin:admin12@' 'http://127.0.0.1:8080/kie-server/services/rest/server/containers/TestContainer/images/processes/NewProj.TestProcess'
  % Total % Received % Xferd Average Speed Time Time Time Current
  Dload Upload Total Spent Left Speed
  0 0 0 0 0 0 0 0 -::- -::- -::- 0
  ============