Unlocking iMovie '09 & '11
In a past post we put QuickTime to work in order to resurrect the creation of still images from video clips. It was a semi-complicated process, but a very powerful and flexible one at that. But you may not want powerful. You just want easy. Here's easy, thanks to the 7.1 update. (You did update iMovie to version 7.1, right?)
Skim your playhead to any point in a video clip that you want to capture as a still image. Right-click and choose "Add Still Frame to Project" from the menu that pops up. iMovie automatically creates a still frame and ads it to the end of your project. (If you have a one-button mouse, hold down the Control key and click to get the same menu.)
If the image was created from an Event library clip, iMovie will auto-apply the Ken Burns effect. If it was created from a clip sitting in a project, iMovie will leave the Ken Burns effect out so your image sits statically in the frame. (This is pretty intelligent behavior. If I am creating a still from a project, I probably want it to be a freeze-frame of the clip I am working on, not a "Ken Burnsed" image.)
Now here's the really handy part. If you want access to the image for other purposes, like adding to iPhoto, right-click on the image that iMovie created in your project and choose "Reveal in Finder."
There it is, sitting in a folder that is hidden inside the project package file. You'll only get JPEGs, but these should work fine for most people. If you want more options for still image exports, go read my original post on this subject.