For some sites, image galleries are an absolute must. Portfolios are the first to come to mind, but galleries are also useful for blogs, e-commerce sites, and really any site that regularly displays multiple images on a single page.
In Photoshop create a Layer Group, and label with the extension ~gallery1, ~gallery2,~gallery3 or ~gallery4. Now either drag the images you want in the gallery into this group or create a new layer and add the image. The size of the top most layer will determine the size on the page this gallery will be. Only visible images will be included, so if you want to save a image but dont want it inlcude, click on the eye to make it invisible.