The heading is wider than both images. If we add the width of the images, margins, and paddings, it sums up to 250px.

Match up the sizes by setting typing in width: 250px; for the heading rule.

<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;
}
</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 width: 250px;.