With Images Gallery you can show your pictures in different categories. An image can also belong to multiple categories.  Gallery creates 3 types of thumbnails to show the pictures. The smallest thumbnail is used when you display a list of categories. The medium thumbnail is used when you display the images in a category and the large thumbnail pops up when you click a medium thumbnail. Then you can also start a slideshow to view the images which belong to a category. The advantage of Gallery is that you can add watermarks (any text as you wish e.g. your website name) to your images.