I've combed the forums, but can't find this answer. I'm new to WordPress, working on a Mac through my Firefox browser. I'm using the K2-lite theme (that permits custom header images). My photo was exported from iPhoto as a jpeg at the exact dimensions specified for the K2 theme (780 x 200 pixels) and the file size is only 120 KB.
The header displays perfectly when viewed in Safari (even though I uploaded through Firefox), but nothing I try in Firefox will enable it to display.
Oh, the blog is: postcardsfromlife.wordpress.com