ToolBox -

This symptom indicates a corrupted download.  Download the image again; when prompted, be sure to click Save, not Open.  Pick a new location and try opening the file from there.

Re-downloading might not resolve the issue if the corrupted file is in your Internet cache.  You can tell if this is the case because the download will complete very quickly (within a couple of seconds).  If this is the case, you will need to clear your Internet cache before downloading again, or try downloading from a different machine.