If you choose Link to originals in the main jAlbum settings (Album » Settings » Pages), your images will not be scaled or modified in any way. They will be used directly in the slide pages of the album. the skin will adjust the slide display to the visitor's viewport, but because the files have not been scaled, the file sizes may be very large, making this setting a poor choice for a web album. However, you can resize your images before using them in jAlbum, in which case choosing Link to originals might be suitable. If you also select HiDPI images, your original images will be shown at one-half their native dimensions, to preserve clarity on high-density displays.
If you choose Link to scaled-down images only, videos will be shown in their native dimensions, or to fit within the image bounds, or to fit within the visitor's viewport, whichever is smaller. If you choose Link to originals, videos will be shown in their native dimensions, or to fit within the visitor's viewport, whichever is smaller.