Blogger's picture icon is good for regular pictures in the post or in the sidebar, but it's overdone for the purpose of putting a photo in your header. You don't need both large and small versions for the header.
What Blogger may give you when you upload the photo to a post (Save it, but don't Publish it!) will resemble this:
<a href="http://2.bp.blogspot.com/_AVxLoZ0mR_0/Rb_W1nLeYQI/AAAAAAAAAIg/qCR5JYhLf6o/s1600-h/dgblues.jpg"><img id="BLOGGER_PHOTO_ID_5025971925707284738" style="DISPLAY: block; MARGIN: 0px auto 10px; CURSOR: hand; TEXT-ALIGN: center" alt="" src="http://2.bp.blogspot.com/_AVxLoZ0mR_0/Rb_W1nLeYQI/AAAAAAAAAIg/qCR5JYhLf6o/s400/dgblues.jpg" border="0" /></a>
What you would need to extract is this:
If you didn't notice on your own, the difference between the red code at the top and the red code above this paragraph is that the -h has been deleted after S1600.
SMALL IMAGE: The url contained in the blue text (marked S400) is the smaller of the images and will need no correction--just the code, such as this, shown above:
Assuming that the S400 is actually small and not just less than the large one, you could enter that code, as shown, into the Photo Url as your Profile Photo.