Header image has disappeared

I’ve recently migrated my blog and now the Header Image is gone. I tried uploading a new one but it won’t show in the blog or in the Header editing area. It uploads into the Media list but not into the Header itself. In the ‘Header’ editing section in ‘Uploaded Images’ (just above Default Images) it shows the radio button active but the image there is just a generic icon for broken image – no actual images shown (there are 2 header images uploaded into the Media list and they usually show in that area).

Can anyone help please? Many thanks.