Are your images raw images? Or Adobe illustrator? Then you may need to reregister
the "Core types bundle" with the OS . You need the Terminal to do this:
Quit Aperture, if it is running.
Launch a Terminal (from the Launchpad, or from the Applications > Utilities folder, or typing "Terminal" into the Spotlight search field)
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchSe rvices.framework/Versions/A/Support/lsregister -R /System/Library/CoreServices/CoreTypes.bundle/Contents/Library
All in one line and without spaces - the only spaces go directly before and after "-R" (the server inserts random blanks into long lines when downloading a post - remove them). Hit return.
Then reboot and try again, if Aperture recognizes the raw format now.