Should not be that difficult. It' all build in.
In QT Player 7 scroll to the frame your want.
Do Copy (Cmd-C). The image is now on the clipboard.
Open an image editor and make a file from clipboard.
Crop/Resize/Save the file.
OR
In QT player scroll to the frame you want.
Do Cmd-Shift-4 and then hit the spacebar. Click when your see the camera.
(Cmd-Shift-Control-4 to copy...



