What you might have heard is that zipping is usually much more effective when compressing text than when compressing images, and some images will not zip at all well. That is because zipping works by finding repeated patterns in the data. Language text contains many such patterns and compresses well. Some images with areas of block colour, like cartoons or logos, might zip well too, but landscapes with cloudy skies usually will not. That is why JPEG, rather than zipping, is more common for images.