There have been several reports of this issue over the lasy while, and we dont know if it's an OS or an iPhoto problem. There is no fix we know about at this time. The workaround is to use the File -> Export command.
and makes the file size 5 times larger
Set the Kind to Current and you'll get exactly the same image you would with drag and drop - a preview of limited quality missing most of the metadata.
This User Tip
has details of the options in the Export dialogue.