Hi Wjchen
I do believe that when reach the step where you want to display the image in the workflow, that you will need to display the image via a view or form with the control on it which loads the image from a SmartObject to where it was saved. This being said I found a historical post which can be used as a solution for your scenario as well.
Please see these Community posting regarding how to Save an image to a SmartObject and then load it to a image control.
Load Picture URL from SmartObject Source
http://community.k2.com/t5/K2-blackpearl/Load-Picture-URL-from-SmartObject-Source/td-p/59501
How to load a picture to a control
http://community.k2.com/t5/General/How-to-load-a-picture-to-a-control/td-p/68126
Note that you will most probably have to do this as you will call the view of form containing the image control during the workflow.
Kind Regards
Raymond