how to determine which image is in use causing conflict

using WP 3.7.3
My site (under construction) has an odd glitch in the header related to background image. On first load http://www.gracechurchhouston.com and with the stretched screen as wide as possible showing outer left/right areas, you will see the nav bar is complete for the header, but the background image which includes the blue gradient as well as a continuation of the nav bar gradient isn’t showing the full vertical depth of the nav bar until screen refreshes. If you do ctrl+f5 on the screen you’ll see the extension of the nav bar area left & right appear narrower vertically than the nav bar itself. It corrects itself once refreshed but i’m trying to determine why this is happening and what graphic is conflicting. Thanks for any help you may have.
Ann