A quick appleScript I wrote to select all images from the current library that are from a certain type.
The type is checked with the extension found in the path of the original picture (original path property of photo object). The below example is to select all NON NEF pictures. NEF filetype being the Nikon Raw format.
tell application “iPhoto”
select (every photo in current album whose original path does not contain “.NEF”)
end tell