Are these images "Raw" images? What is the make of your camera?
Check the version of the Digital Camera RAW Compatibility Update that is installed. The version of the raw support is shown in the About panel: from the main menu bar:
Aperture > About Aperture
Download the latest Raw Update from here:
If you have the latest raw support installed, but Aperture does not recognize the format, try to register the raw support for your camera with the launch services db, see this post by Alan Roseman:
Re: Aperture 3 preview of raw file greenish
But also read the following post, that explain how this is supposed to work and to correct the typo in the command 😝
In short: Quit Aperture, open the terminal, paste the follwing text in (all in one single line) and hit return; then reboot the system.
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchSe rvices.framework/Versions/A/Support/lsregister -R /System/Library/CoreServices/CoreTypes.bundle/Contents/Library
Regards
Leonie