Hello, Steve.
Every step of a map you have drawn as a process definition consists of a different WorkItem. Every task correspond to a WorkItem that must be completed by a participant.
When a task is concluded, tha WorkItem information is stored in AWS database and no longer used along the rest of the process.
So, if you´re setting the value for a variable in the first form/step and want to use a few steps ahead, WorkItem.SetFieldValue will do you no good.
What you have to do is to set up a "global" variable, that will keep it´s value along the hole process. By doing this, you assure that it will have the value you need by the end os the process, which I assume to be an image name or an image reference path.
Try Process.SetFieldValue(VariableName,VariableValue), where VariableName will be the name of the variable you´re creating, and VariableValue will be the path to your image. The PROCESS object is global for the hole instance, that is, differente instances may have different values for that variable.
Let me know.
Regards.
Daniel Maia