IG will always crunch your images through its own "optimization", especially for the preview images and low-res versions, so this is ultimately a moot point. essentially once you upload, your original source image is gone and the service is really not meant to distribute high-quality artwork. You can improve results by adhering to their default resolutions, which are all based on using 1080 pixels for height and width, but that's pretty much it. If that's to lo-res for you, then simply use Flickr or another service that also retains the original image.
Mylenium