You'll need to specify height as well, or the height will default to 48 px.
You should check out this blog: http://www.markeverard.com/2014/02/13/image-resizing-in-episerver-7-5-cms/
It might just be exactly what you need
Admin -> Scheduled jobs -> Clear Thumbnail Properties
I have an ImageFile from wich I'd like to generate 300px wide images. The problem is that the images that I get, they sure is 300px wide, but most of the image is wite space. The image itself is just about a ten pixels wide. Does anyone know what could cause this?