Check the zoom factor on the import. It may be zoomed in too far. What are the pixel dimensions of the image. If it is too big compared to the stitch count, it may cause a similar issue. The best images have a 1 pixel to 1 stitch ratio (or as close as possible).
I tried zooming in and out and resizing the image, but I still can't import it. I was able to import an image I had imported previously, but not something new. They're all simple jpg images.