Snippet for a SharePoint Script Editor webpart, to show an image and on a click show the image

ShowImageInPopupDialog.txt

<script type="text/javascript">

var dlgOptions;

function setDlgOptions(pictureUrl) {
	var htmlFragment = document.createElement("img");

	htmlFragment.setAttribute("src", pictureUrl);
	htmlFragment.setAttribute("width", "800");
	htmlFragment.setAttribute("height", "600");

	dlgOptions = {
		title: "The Image",
		width: 800,
		height: 600,
		html: htmlFragment,
		showClose: true
	};
}

</script>

<img src="https://your-tenant.sharepoint.com/sites/img-tester/Shared%20Documents/Sonnenuntergang_mit_Segelschiff.png" 
     width="400" 
     height="300" 
     onclick="setDlgOptions(this.src); SP.UI.ModalDialog.showModalDialog(dlgOptions);">