Problem uploading new header image – File Empty

Hello,
I would like to upload a new banner on my website however when I try to customize the appearance then add new image as banner I received the following message on the right side:
?File is empty. Please upload something more substantial. This error could also be caused by uploads being disabled in your php.ini or by post_max_size being defined as smaller than upload_max_filesize in php.ini.?
When I tried to upload the same banner in JPG or PNG than the one already one the site I get the same issue. I tried in a different browser and with the image in different format like .PNG but still not working. Could you help me with that please? Thanks so much in advance! M