Loss of sharpness in jpg imported as header in twentyeleven

When importing a jpg into the header position (jpg created in indesign at 1000 px wide) for the twentyeleven theme, the photos are not sharp. what is causing this and how do I fix it?

Before hitting the button “use this media”, have you checked “Full Size” in the Size option?

It’s basic, but at times one skips over this things without realizing.