This is amazing! Finally! After years of having only printed and extremely low resolution versions of this promo image, someone found a copy of the original. This is at glorious 1771 by 2500 pixels!
Backstory time. When Star Fox Assault was being released, Nintendo and Namco provided promotional images to promote the game. These were sent out to websites and media outlets in high resolution so they could have visuals big enough to print or use in online graphics. This particular one, of Krystal holding her gun over her shoulder, was only ever publicly published online (to my knowledge), on the official Japanese website for Star Fox Assault, and at a very low resolution. However, it did exist in higher resolutions. The various Star Fox Assault Player's Guides in Japan featured this image in much higher resolution and I even found it used by this French publication.
But what about America? Well, I actually managed to obtain the official press kit for that year's E3, which contained a lot of high resolution renders from Star Fox Assault. But that one? That one wasn't in there. I guess Nintendo decided to keep it out of the North American market, perhaps because it was considered too sexy. Whatever the reason, it was never published anywhere that could easily be obtained. I floated the idea of contacting the publishers of the various Japanese Player's Guide companies to see if they would send out the image, but I was never able to get a good Japanese-speaking contact.
Then, a few days ago, this topic on Reddit showed up. I was surprised to find it to be the full resolution. I had thought it was my original ripped image that I had cleaned up and removed the background from. But it was much too detailed. An artist's reproduction, perhaps? No! This was the original, or at least a resized version. The original post creator pointed me to this page on the Star Fox Wiki. You can see that it was retrieved directly from "Nintendo of Europe", which probably means their media distribution website. The real questions now are, who had access to that media site, and what other Star Fox materials can be obtained there? I have already confirmed that their version of Krystal with the minigun is higher resolution than my already high resolution version.
All this was done last year, in January 2019! Amazing that so many years later, we're still discovering new things.