There’s a bunch of posts about this already if you do a search on the forum or elsewhere. First, check in your preferences under OpenGL to see if ‘Use Maximum Texture Size’ is checked.
Second, SU maxes out image resolution so it doesn’t help that your image has 21,000+ pixels when SU won’t show more than 1024 x 1024 pixels. Not enough I know but that’s a different conversation. So to resolve, crop in on your image as close as you can before import. If you need such a huge area, tile it. Break it up into a few pieces, save as individual images and import each and line up to increase resolution.
Without ‘Maximum Image Size’ selected
With ‘Maximum Image Size’ selected
Image brought in as tiles:
See resolution difference between before and tiled: (faint grey difference between images)