This is accomplished by setting ALBUMS/Preview Style to "Fill". When doing so, the album title and description text field is removed, and your image is scaled to fill the album element. To create images at the exact dimensions required, set the PLAYBACK/Startup parameter to "Open Gallery" so you can see the album dimensions (you can always change it back afterward when you're done). The dimensions are shown in the live preview.
If you are using SlideShowPro Director, you can either let it create album preview images for you (using content that's been uploaded to an album), or you can upload custom album preview images for your album to use.