Oxygen Theme: Header image

I am a complete beginner to all of this, so be patient. Was using WordPress.com version of Oxygen where the header image was recommended to be 940x150pixels. WordPress.org version only recommends 400×100.

Oxygen, however, claims to offer a flexible header meaning none of this would matter and I could carry on using my larger banner image. Unfortunately it doesn’t seem to be working and refuses to give me a bigger image than 400×100. I’ve seen quite a few posts on the subject, but haven’t been able to gleam anything from them.

Any help would be welcome. If it involves altering CSS, please be detailed (as in where/what exactly I have to change).

Cheers,

Adam