Go to the given site, then press Ctrl+Shift+C or left click and inspect element.
Left click on the code, edit as html.
Press Ctrl+A to select everything, delete the code and replace it with this:
<html><head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
img {
width: 100%;
height: auto;
position: absolute;
margin: auto;
left: 0;
right: 0;
top: 0;
bottom: 0;
}
</style></head><body>
<img src="INSERT IMG URL">
</body></html>
Paste the link of the desired image inside the " ". (You can upload the image to Imgur or any image hosting site, copy it's link and paste it here.)
Exit the html view mode.
Click the hearth icon to save the site to your speed dials, and scroll between the images until you find the one you uploaded.
Works in Opera GX as well.