Welcome ,I will give a full and clear answer to your question, please make sure to reply to any questions I have. They may be essential to my reply.
Hi are you using iphoto ?
Ok lets do some repairs .
A safe boot can help ,Turn off the Mac .Power on the Mac .Hold shift on the keyboard using a USB keyboard , while powering on the Mac and booting , boot to safe mode.Once booted to safe mode , restart and boot as normal .Full details here :-http://support.apple.com/kb/ht1455
ok after that, lets do some iphoto house keeping .
You may need to rebuild your iphoto library!(THIS SHOULD NOT DELETE ANY PHOTOS)Hold down the CMD & Alt while opening iPhoto and select rebuild library...This should get you up and running again , or you may be presented with the following options :1) rebuild the photos' small thumbnails2) rebuild all of the photos' thumbnails (this may take a while)3) rebuild the iphoto library database4) recover orphaned photos in the iphoto library folder5) examine and repair iphoto library file permissionsPlease carry out the options for repair / rebuild of the iphoto library in the order below:3) rebuild the iphoto library database.If needed.5) examine and repair iphoto library file permissions.If needed.2) rebuild all of the photos' thumbnails .
Does this help the issue ?