Read more about The designer’s guide to image slicing in Photoshop at CreativeBloq.com
When working with images on the web, there’ll be times when you want to load a large image, either as a main image on a page or for a background. The downside is you will immediately run into several problems. First off is the actual file size. Second is the resolution. Third is compression. If the file is large it will take a long time to load, especially for users with a slower connection. You can reduce the file size with compression but you can only go so far before image quality will begin to suffer.