To create thumbnail image with CSS, use the border property.
Example
<!DOCTYPE html>
<html>
<head>
<style>
img {
border: 2px solid orange;
border-radius: 3px;
padding: 7px;
}
</style>
</head>
<body>
<img src = "https://www.tutorialspoint.com/videotutorials/images/coding_ground_home.jpg" alt="Online Compiler" width="300" height="300">
</body>
</html>