Basic stuff … header image won't display

I’m new to WP and seem to having a very basic problem.
I’m using the free theme Blaskan and I’m trying to add a custom header.
I created a jpg file to the recommended pixel size which I’m very familiar how to do and saved it to my computer.
When I click the Appearance > Header links I get a page to upload my custom header.
I choose the file from my computer and click the upload button.
It uploads and shows the filename in my media library but the image won’t display. I get the blue question mark and taken to the ‘crop image’ box again with blue question mark.
Any idea’s? It seems like this should be a very straight forward thing to do so why won’t it display?
My site is unpublished as this is my first page.
Let me know if you need more info. Thanks.