Finally, let's center the text.

Set the text-align property to center.

<html>
<head>
<link rel="stylesheet" href="style.css">
<style>
img {
width: 100px;
height: 100px;
margin: 10px;
padding: 5px;
border: solid olive;
}h2{background-color: lightGrey;margin: 10px;padding-top: 30px;padding-bottom:10px;width: 250px;border: solid olive;text-align: center;
}
</style>
</head>
<body>
<h2>Photos contributed by our users</h2>
<img src="https://mimo.app/r/tree.png">
<img src="https://mimo.app/r/trees.png">
<br>
<img src="https://mimo.app/r/starrynight.png">
<img src="https://mimo.app/r/lighthouse.png">
<br>
<img src="https://mimo.app/r/pathway.png">
<img src="https://mimo.app/r/ridge.png">
</body>
</html>]]>

Make sure to type text-align: center;.