You are going to need some html to control the size of the initial image:
<!DOCTYPE html>
<html style="background-color: black;">
<head>
<title>ImageView</title>
<meta name=“viewport” content=“width=device-width, initial-scale=0.86 minimum-scale=0.25”>
</head>
<body>
<img style="width: 100%; height: 96vh;object-fit: contain; object-position: ;" src="myImage.png"/>
<body>
</html>
If you have a file: myImage.png in your assets
Create an html file with the above, call it myImage.html and upload it to your assets
Set/call webviewer url to “pathToAssets”/myImage.htm
The image will fit the screen and you can zoom