Change behavior of image tag on my site

I would like my users to be able to simply type the name of a directory and image inside forum, news, and article posts to reference an image.

Currently to include an image in a post requires long form urls - example: [i m g]http://www.website.com/imagedirectory/imagesubdirectory/imagename.png[/i m g]

This requires effort i feel effects the use of images in a negative way. I would like a predefined the website and main directory leaving the users only to input the source directory and image name. Example [img]imagesubdirectory/inamgename.png[/img]

Basically predefined for the [img] [/img] tag would be http://www.website.com/imagedirectory/

if there isn't a way to define the directory for the images is there a way to create a site image button they can click to add content hosted on my site instead of having to type in long form urls?