Theme header red x

Hello.

I recently saw red x appear in my header instead of my header images and changing the image was of no help. So I backed up my files and re-installed the whole wordpress setup. But even before I brought my backed up files to my new website, red x again appeared instead of my original header. I used custom-made theme before and now I’m using Nirvana theme. But problem happened again.

What might be the prolem? and what should I do with this problem?