Set a visible 4px border around all images.

<html>
<head>
<style>img {
border: solid 4px black;
height: 130px;
width: 130px;
}</style>
</head>
<body>
<img src="https://mimo.app/r/shuttle.png">
<img src="https://mimo.app/r/moon.png">
<br>
<img src="https://mimo.app/r/moon2.png">
<img src="https://mimo.app/r/moonlanding.png">

</body>
</html>]]>

Make sure to type solid 4px.