Getting rid of the -300×300-resizing and using w=300 instead?

Hello everyone.

I’m not sure if my question is common, but I’ve been searching to set this up for myself without any luck so that’s why I’m turning to this forum – hopefully you guys can help me out.

I’m a blogger for wordpress.com, to which we only have certain settings -but then I run my own blog at so-random.org to which I would like to have some of the stuff from the dashboard settings of wordpress.com. I’ve been wanting to have this image setting (probably new, since it works on wordpress.com blos) which dynamicly resizes the images after the setting looking like this: /wp-content/uploads/2013/01/25/header.jpg?w=300. My current image resizer creates medium and large images like this: /wp-content/uploads/2013/01/25/header-300×300.jpg.

I hope you understand where I’m getting at! I have the latest WordPress version installed on my own hosting, so it must be some kind of plugin or code which makes this work? Let me know.

Forever grateful!