If its a psd file, put it into iPhoto first. Simple drag and drop of the psd file over iPhoto will start the import. Then in iMovie, add it to your movie. In the main iMovie window there should be media, click on that, then on Photos at the top right. Should be in the list of photos.
This is iMovieHD, since that is what I have. Might be different for you if its and older iMovie.