adding custom image

Whenever I try to upload my custom header, I hit upload button, but nothing happens.. its blank…
I have even cropped my custom header to its correct size according to the theme.
I have searched many forums here but they are very old and closed.
Can someone help me figure out what I am doing. I am very new to wp but quick to learn.
I have tried many themes .. nothing works..

[No bumping, thank you.]