when I send some URL to someone via Viber the Viber load the URL and displays image representing the HTML page.
How to tell the HTML page what image it should be?
You should use Open Graph protocol. You can simply add this snippet to your page <head>:
<meta property="og:type" content="website">
<meta property="og:site_name" content="Название сайта">
<meta property="og:title" content="Заголовок">
<meta property="og:description" content="Описание.">
<meta property="og:url" content="http://example.com/page.html">
<meta property="og:locale" content="ru_RU">
<meta property="og:image" content="http://example.com/img.jpg">
<meta property="og:image:width" content="968">
<meta property="og:image:height" content="504">
<meta name="twitter:card" content="summary_large_image">
<meta name="twitter:title" content="Заголовок">
<meta name="twitter:description" content="Описание.">
<meta name="twitter:image:src" content="http://example.com/img.jpg">
<meta name="twitter:url" content="http://example.com/page.html">
<meta name="twitter:domain" content="example.com">
<meta name="twitter:site" content="@">
<meta name="twitter:creator" content="@...">
Image size should be 968x504px. Read more here
Cheers!
I believe that Viber takes the first picture it finds from your website (backgrounds not included!). I have made some tests and have found out that if you place an image in a comment section at the top of your HTML - <!-- <img src='myImg.jpg'/>--> - it will also accept it as a first image file in your source code. Thus you can put whatever image you like.
来源：https://stackoverflow.com/questions/34941283/how-viber-select-the-image-to-display-from-html-page