An easy way to Un-Burns images inserted into iMovie 3, which allows you to set the length of the image clip easily, is as follows. Set the time of the Burns effects to minimum (0:03) set the zoom to 0 for both the start and finish points (1.0 zoom level). Drag in the required clips.
This should render quickly, I dragged 18 images simultaneously on an old G4 400 and it finished importing / rendering in under a minute. Once done, move the play head to the start of the first clip, select "Create Still frame" from the menu (or hit Command + Shift + S), then press right three times (to go to the next frame) ... repeat the Still Frame command. When you've finished, delete the rendered clips and in the clips window you will have all your still images.
This has a number of advantages over other methods I've seen. Firstly it allows you to easily change the length of the still images (double click) and it keeps the size of the project down, as it just extends the play time of the still image rather than having a long rendered DV stream sitting at about 3.5MB a second.
[Editor's note: There's another method detailed in this hint (as well as a third method in the comments to the hint), but this version is unique enough and has merits of its own I thought it merited it's own posting. I much prefer having non-rendered, length-adjustable still clips as provided by this hint...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030207070603841