I struggled with this also, but I've found this is solved when you have a non-retina image named image.png and its retina counterpart is named image@2x.png. So to fix yours you'll need to name the first "Default-Portrait-1024~ipad" and the second "Default-Portrait-1024@2x~ipad". The part preceding the "@2x~ipad" needs to be the same for it to recognize them both. It's dumb and can make the naming confusing, but that's how it works for now. I'm not sure if this functionality is intentional, since it doesn't seem to work this way for any of the icon images...
Hope this helps!