I'm sorry to hear that you're unable to open images from your EOS Rebel T3i.
I have to agree with Scatterbrained in this case.
It sounds like your images have been taken the RAW format. To check this out, take a look at the file extension. Does the filename end in ".JPG" or ".CR2"?
What version of OS X are you using, and what version of iPhoto?
For the record, iPhoto and OS X are perfectly capable of editing (T3i) RAW images, provided you're using a fairly recent version of both iPhoto and OS X, and have the latest Camera RAW Compatablity updates installed.
For example, iPhoto '11 edits the RAW files from my T4i just fine:
I don't recall offhand when iPhoto first started supporting RAW, but from the look of the screenshot included in this KB article is anything to go by, it was a long time ago: http://support.apple.com/kb/HT2297 (that's from between OS X 10.3 and 10.4 -- or late 2003 to late 2005ish).
To be able to help the OP, we really need to know more about what version of OS X and iPhoto....