This can be handled a few different ways. The quickest way is to set CONTENT/Scale to "Crop to Fit" or "Crop to Fit All" (the latter uses the full width/height). This will scale your content so that it fills the space, then crops out the center. If using SlideShowPro Director to load content, it will use the focal point assigned to the image to determine where to crop the image (see "Focal Point" in the Editing content overview in Director's User Guide for more information).
The other option is to create your own images that are the exact same size as SlideShowPro's Content Area. You do that by using the width / height displayed in the top-left corner of the component's live preview as your target dimensions when creating your images.
Adjust the width/height of SlideShowPro until its Content Area is a square with equal dimensions for width and height. This way the largest side of your portrait / landscape images will be the same.
If you'd prefer more of a visual buffer between images and the navigation, and are creating your own slideshow content, simply create content that is a little smaller than the Content Area's dimensions, then (optionally) align content with the top of the Content Area (done so through CONTENT/Align).
If you'd rather not create all new images, you can force them smaller with the CONTENT/Scale Percent parameter. The value is a percentage, which is a scale of 0-100 in the ActionScript 2 version and 0-1 in the ActionScript 3 version.
If you are using SlideShowPro Director, you can make Director publish images a little bit smaller than the Content Area by modifying the aforementioned CONTENT/Scale Percent parameter as well.
See the Creating slideshow content: hyperlinks instructions in the :stapi: User Guide.
The first and most basic way is to lower image quality, decrease size, or both. If you are using SlideShowPro Director to publish content, you can control the quality (and thus file size) of the images it creates through the DIRECTOR/Large Quality parameter.
Another way is to pre-fetch slideshow content into the browser cache when an album loads. This way by the time a user gets to an asset, it may have already been cached, and thus loads immediately. You can enable this functionality through the OPTIONS/Cache Content parameter.