I was struggling with a custom header also. A co-worker suggested designing the header in program like Publisher and then save as a picture. I did that, and then they took the picture file and put it in photoshop to set the right size. Then saved it again. I then uploaded the photo as a photo for the banner. Left the rest of the banner alone. It looks good!! IT’s a bit of a work-around but I was able to have exactly what I wanted.