Hi Nathan
What might be happening here is that you are actually selecting the wrong output option in the export menu. To see your Photoshop file on screen via your Intensity, you need to select File>>Export>>Blackmagic image export and this will send your still frame to the screen. When you hit this option, you will have to select a resolution and output channel that matches your image, to display correctly.
To update your output each time you make a change to your image, just use the Export>>blackmagic option again. It may be worth binding this action to a hot key using the actions pallete.
If you use the send video preview option, you will get a device not found error. Hope this helps!
Callum McLay
Technical Support Consultant
Blackmagic Design EMEA