Images don't have their own separate numbering. All Wordpress pages are stored in Wordpress database in records called "posts". Each page, for example home page, login page have unique ID number in database table. When you are adding new image, it gets first available ID. Every image occupies three separate post (ID of one of this posts becomes image number), so if your first image has ID 203, next will be 206 and so on. These numbers should be rather assumed as unique image identifiers, not consecutive numbers.