Sight requires 580x150px header images. The URL of your image says “cropped-headerwp2.jpg”, so the image you created and uploaded wasn’t in the right dimensions.
But inserting such text as an image isn’t good practice anyway. If you don’t want all that as your tagline, why don’t you add it in a Text widget at the top of the sidebar?
And even if you don’t want it in the sidebar, it’s probably possible to reposition the Text widget via CSS editing.