t.rb

participant = ImageParticipant.new(opts)

# with error when reply_to_engine is not called
participant.should_receive(:reply_to_engine)

# if you know how the workitems instance should look like when passed to reply_to_engine:
participant.should_receive(:reply_to_engine).with(expected_workitem)

# if you don't care if reply_to_engine is actually called
participant.stub(:reply_to_engine)

# and now: call the method to be tested
participant.consume(workitem_before_consume)