How can I take photo from an iMovie clip?

Put the cursor on the chosen frame in the event or project, right-click (control-click) and choose "add freeze frame (to project)". It automatically adds the frame as a still to your project. If you want the still as a jpeg file for other uses, right-click on the added still and choose reveal in finder. There's your jpeg file ("stills" folder within the iMovie project). I normally import a copy into iPhoto for diverse uses with other programs.

Have fun with iMovie