Box is correct. If the image is left as an image, it will remain visible when you are editing a group or component. If you import it as a material (or import as an image and immediately explode the image which converts it to a material) the material won't be visible if it is outside the context of the group/component being edited.
I've seen some tutorials showing the importing of images to use as references where the advise it to immediately explode the image and then group the geometry. I've never understood the reasoning for the added steps, especially if the image is only a reference and will not be used as a material.