When I render an image, the "Image" menu has an option to "Edit Externally" but I can't get it to work. Would love to render, then edit in Photoshop. Can anyone help? I've tried setting the "image editor" in user preferences.
You have to set the associated image editing program in User preferences > File first. Next, make sure that the image is saved to disk (you can use F3) and it should work.
When saving, use Save As Image and untick Save as Render and Copy but keep Relative Path.