Just save the images to your phone and then use the app to access the images on your phone.
I personally just use Dropbox to download them to the phone so I don't have to plug anything in and it just syncs with the cloud.
You can also email them to yourself and save the images to your phone that way.
But the idea of IG was supposed to be that it's live images in the moment that you're taking with the phone, they aren't supposed to be pro images, although now pro's do use the system, but that's why you can't just upload your images from the computer, you need a smartphone/tablet.