How to:

1. Create an Action with two inputs, the first is a Text field and the second a BinaryData, this is where you will have to process the image, use the Text field to send Id's or extra information you require, a JSON for example.
2. On the action where you want to trigger the image download drag a Javascript block and in it create an output Object with something like this:
   $parameters.Callback = $actions.Callback;
3. Drag the TriggerRequest action, and fill in the inputs.
4. And you are done, check the demo for a better example.