I suspect the reason you are getting artifacting when resizing it, is because the image is actually showing the wrong dpi.
I can almost guarantee that the image in question is actually the right size, just the header has somehow got messed up, and dpi reduced from 300 to 96, and, this has therefore scaled the dimensions up. To get it back to the right size, you need to reduce the size, and increase the dpi at the same time. This should retain all the information within the image, without actually physically changing it. Just reducing the dimensions, it will remain at 96dpi, and will be terrible quality scaling.