okay, adding the line in the CSS definitely helps. The nav menu now opens below the header image like it's supposed to and the header image stays put. But now the header image just slightly overlaps the nav menu when at normal size. (It would look like it overlapped more if I didn't have whitespace added onto the bottom of the image itself already.) I might just add some more whitespace unless someone knows how to fix that overlapping behavior.
Thanks for the help so far, though! :)
