Additionally, the number and degree of brightness of colors in a scene can increase or decrease file size. If, for example, you shoot a subject in black and white, the file size will be a smaller than if you had taken the photo in color.
Editing and resaving an image can also significantly increase or decrease the size compared to the original file.
The average size of a five megapixel color image file is between 2-2 1/2 megabytes when shot at JPEG fine (least compression). That means you can get about 280-300 onto a CD that has a 700 megabyte capacity.