If the caption is placed on either side of the slide image (see Layout » Slides » Slide Page Caption), the width of the caption area may be specified, and the user may choose to have the image remain centered on the page or to center the combined image and caption.
If the caption is placed on either side, it's often best to change the alignment of the title, comment, counter, and metadata from the default center to right or left (caption on the right, align the text to the left, for example - see Layout » Slides » Slide Page Caption).
Using side-loaded captions requires some consideration of page width. If images are scaled to 640px wide, the caption is 150px and is placed on either side, and the image is centered, the page width (including image borders) can easily exceed 1000px. Bear in mind that many laptops have monitors that are only 1024px wide, so a page that's wider than that will produce horizontal scrollbars unless one of the skin's responsive slide modes is used. In that case, the slide image will be reduced so that the entire page is visible within the viewport. This may result in a very small image. The largest page width in an album can be determined by opening the system console (F7) after the album has been generated - the skin places a message in the console that indicates the size of the widest slide page.