Does anyone know?

how to add and change the size of the picture in the header here at blogger? I have tried the 'shrink to fit' etc and been on a couple of video tutorials but my personal photos seem to be too large and the free banner I have had as a header just doesn't fit properly. I would welcome advice here. I guess there is some editing of the Html and I have done this once (having taken advice from a blogger help forum and taken out the width etc numbers). I really want to change this and get it right and I have a feeling that it can't be that hard!! Help from those who have done this successful would be so appreciated. Thank you.