Help in putting up a header

[imperial_dragon]

Hi. I's like to put an image in the header for my journal but I am battling to find any information I can understand on how to do this. I have no experience in using sites like Dreamwidth and don't know anything of CSS or where you would put it so -

I finally worked out how to use an image I uploaded to the image hosting for the header but it only shows the very top of the image, which I presume means that the image is too big to show fully. But I can't find anything to tell me how big the image should be or if there are any other ways I can change the way it shows. I'm using Elegant Notebook for Gold Leaf by rosecarmine

If anyone can help I would be very grateful but keep it basic please.