Thank you for your post.
The "Calculation result must be text" on the Message input for a Show Custom Dialog script step. There is no way to display an image using the Show Custom Dialog script step. Depending on your needs, you might look into creating a layout to display instead of a Custom Dialog.
I hope this information is of use!
Thanks yes I suspected new window - new layout - might be the way to go. thank you
that, or I've seen a lot of people using the pop-over to "simulate" a dialog. It floats above, just as a dialog would.
The difference being the types of new window (modal perhaps) you might get.