I just tried it myself for the first time...video/pictures were shot and edited in about 5 minutes. Upload to YouTube in HD didn't take that long (but my video is short).
I haven't had a chance to try this yet, but if a picture is used in iMovie at the end of the video, there doesn't seem to be a way to add a transition at the end....heck, I'm not even sure if you can do it if a video clip is at the end of the strip...I hate the way it just cuts off at the end...would rather have it fade out.
http://www.youtube.com/watch?v=IJ73926BRog