Currently default image functionality is only supported for sections and category items, who have "Image" tag. In case if image wasn't yet uploaded, but something still should be shown on Front-End, then default image is right for this case.
I propose we add "default:path/to/default_image.jpg" additional format and it should used this way:
<inp2:widget_Field name="Photo" format="resize:100x120;default:img/default.jpg"/>
This new default image is only useful in cases, when original image (which is not uploaded) should resized. This approach guarantee, that default image will be resized as well.