Yeah me as well! When you use the knife button you should select the square around the image and and let go the mouse click Where did you made the download of GIMP?
Don't you have another program to cut images? Normally I never use GIMP to cut them! OR you can open a new image and open the image you wanna use appart and paste on the black image! ^^
You know some people have some troubles with the new GIMP 2.4, maybe yours also had those Glad it's working now! Most times it's a problem that comes with the program!