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 400x100.
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 400x100. 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).