How to make a close button "X" stand out from a image

I have an image looks like satellite map where I want to put a close button at the top right corner. What I have now is just a span with "X".

It's an 100% overlay on a popup so clicking outside would close the popup where I only want to close the overlay.

But because the image itself can have any color, thus no matter what color I make the "X" to be, it's not always very visible to the user.

Is there any (CSS) tricks that can make the close button stand out? I've heard about box-shadow but not sure how it can help in this case.

Having a dedicated header to just hold the close button feels feels too much.