The custom image in the browser tab is called a favicon
To replace the standard W icon on WordPress sites, you can do the following.
Modify your image to be 16px x 16px. Save it as favicon.ico
Your image may not save exactly in that format. If the size changed but your image editor did not save it in the desired profile. On a mac, you can select the icon and do a rename with a file change. You will get the following warning, confirm.
But a good practice is to just convert the .png that you create in Photoshop (or other image editor) to the .ico format. You can do so using free sites such as www.convertico.com
Now that you have the image ready. Upload it to the main folder of your site AND the main folder for the theme you are using in wordpress. In my case I am using Buddymatic
Now go into the Header or header.php
Below the <head> tag, which in this case looks like <head profile=” ”
You will need to insert this following line of code (this image above shows it already in).
Save your changes and then refresh the page that you are viewing your site on.