Twenty Fifteen theme header image

Hi, I'm feeling somewhat dumb about posting this question because it seems like it would be a verrrry simple thing to do. But.. When I make the header image for the 2015 theme it tells me that it should be 954 × 1300 which is a very tall image but OK, I did it anyway, placing the text of the logo near the middle of the image. That made the text of the image show very low in the sidebar and off to the left. I tried adjusting the placement but it never came in right and sometimes would just disappear from the tablet view completely.