How to customize sydney child theme header suggested image

hello i am learning wordpress developpement with sydney theme but i have make a website when i paste the link on facebook for someone it schow an image call header.jpg located in
domainname/wp-content/themes/sydney/images/header.jpg
i went to appearence then header image and added new header image but it still showing the same image
when i paste the link on facebook or skype for someone.
please how to change this image and set my home image

Hello there,

The changes might not instantly be recognized by certain applications. Please try again later. Meanwhile, try to use this plugin: https://wordpress.org/plugins/facebook-thumb-fixer/

Regards,
Kharis

i install what you sent me but it still doing the same thing
i think it is not for facebook only because it does the same with skype
if you click on this link you will see the image that am talking about
http://www.secretelectron.com/wp-content/themes/sydney/images/header.jpg

and if you paste the link http://www.secretelectron.com/
in facebook chatbox as you want to send message to someone it will show the image same with skype

Hello there,

To override the default image, activate the Facebook thumb fixer plugin. Then from your Dashboard, go to Settings > General > supply your image URL path into the field that says “Default Facebook Thumb”. You also need to supply your Facebook App ID and specify the homepage object type (I recommend website).

Doing this will produce an og:image meta tag value with an image you specified. This is used by Facebook and chat apps like Skype as an image preview.

Ref:

Regards,
Kharis