Your second suggestion worked. I changed the alignment to “None” and everything worked. I could get rid of the white line as well. So, if someone wants to use a larger, distinct image on all of the pages: choose the custom header option; check the custom header box; place the header image in the content section of the header settings; and select “None” for alignment when importing the image. If you want to remove the white line, add the background color under the content section. Keep in mind, however, that you will have to add the page titles and icons to the image manually. You can do that by switching to the standard header option and grabbing a snapshot of the page title.
Thanks for your help!