An image with unwanted posterization exhibits banding or sudden shifts in color. It can occur with improper image-editing or poor photographic or scanning techniques. Converting an image to GIF or other formats with limited color palettes can introduce posterization artifacts and unwanted color shifts.
On the other hand, images may be intentionally posterized to give a photographic image a stylized line art or impressionist painting appearance. Once a complicated darkroom process, today intentional posterization can be done in just a few clicks in Adobe Photoshop, Corel Photo-Paint, or other image-editing software.
In the sidebar, the two images on the left half are a digital photograph and a gradient fill with no posterization. After posterization, the banding or color shift is obvious in the images in the right half.