How can I locate in iPhoto an image selected in the Finder?
Here is a basic automator service that will allow you to select an image in Finder and then start the service to find it in iPhoto. It worked fine on my Snow Leopard machine.

HTH





EDIT:
Changed the code slightly to make it behave better and make it go into Edit mode. Here is the changed code. Open the Automator Service file, and copy/paste it over it. Save it and it should take effect immediately.

on run {input, parameters}
    set fPath to POSIX path of (input as string)

    tell application "iPhoto"
        activate
        select album "PHOTOS"
        set photoList to every photo whose image path is fPath
        select item 1 of photoList

        # Pressing Enter key goes into Edit mode...
        tell application "System Events"
            keystroke return
        end tell
    end tell
    return input
end run