Set the width to 100px.

<html>
<head>
<link rel="stylesheet" href="style.css">
<style>
img {
width: 100px;
height: 100px;
}
</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 pick width followed by : and then 100, px, ;.