>> Please let me know when it's done. << - Jira is the way for you to raise bugs and get notifications when they are fixed. I am not going to send you a personal letter when something you did not report properly gets fixed :)
What distinguishes Multiple instance subprocess from a plain subprocess are the collection expression and the variable name properties. If you do not set those designer will re-parse your subprocess as an embedded subprocess (default). The "could not find process image" is a warning by the visual validation, but you can store your process w/o ever creating a process image. To generate the process image just click on the PNG button in the designer footer.
So I don't see this particular issue as a bug, but Instead of just saying that you don't know how to do something (open a Jira), a simple google search would take you to https://issues.jboss.org/secure/Dashboard.jspa where you can create an account and start being part of community by contributing bugs that you find :)
Thanks.