Header image

So I have ran into 2 problems the first being this little rectangle on the bottom of the menu bar It appeared after I did something but I cant recreate it. The second is I can not figure out how to add my own image in the header below the main menu bar. I have found the after nav in the main css but I cant find where it is in the other files. I would like to use my own picture and not a slider similar to this thank you!