Sometimes when you are downloading images from Cisco corruption may occur. Of course, this is most likely to happen at 0200 in the morning when you cannot download a new copy.
When you download the file from Cisco make a note of the MD5 sum on the download page.
On OS X you can open a terminal windows and do the following
gf:ACE gregferro$ md5 c6ace-t1k9-mz.A2_1_2.bin MD5 (c6ace-t1k9-mz.A2_1_2.bin) = 80e59bca28c16803fdeb8d402a211f8f gf:ACE gregferro$
As long as they match, you have a good chance that the file is not corrupted.
A quick google brings up plenty of Windows tool to do the same job.