This is a pretty cool function. Sometimes the default Medium and Large sizes when inserting an image into a post just isn’t enough range. Maybe you have a special size you use in posts but don’t want to change your Medium and Large file sizes just for that? Well this snippet will allow you to add another image size to the WordPress drop down menu, when inserting an image into a post.

To add more sizes just keep adding more add_image_size’s and add them to the $addsizes array.