Img block percentage

Well, I guess mostly this is a tryout as this is my first pull request ever ;)
For a client, I had to add the possibility to set the percentage of the image based on width (well, mostly to set it to 100%.). As I did recieve that request quite often, I figured I just try out to make a pull request. I won't bother if its declined though, as I don't know if this really is useful or placed at the right position.