There are many different ways you can approach creating a photo gallery in ExpressionEngine. At the most basic level, you have a choice between each channel entry containing only one photo (and a description) or each channel entry containing a set of photos. If you allow only one photo per entry, you can use categories to organize the photos into galleries (and you can even include the same photo in more than one gallery). If you allow multiple photos per channel entry, each entry represents a photo gallery by itself.
One way to accommodate multiple photos per entry is to create as many custom fields as the photos you think you will have in a gallery. For example, if you know each gallery will have a maximum of 20 photos, then you could create 20 custom fields for the photos and 20 custom fields for the descriptions. This solution works, but is not the most flexible (that is, to add 21 photos to a gallery, you would have to modify your custom fields and your...