I made this a long time ago but hopefully this should still work. You will need to inspect element and replace the entire HTML with this:
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<img src="INSERT IMG URL">
<!-- After making the thumbnail, switch the current thumbnail to the generic thumbnail (the one with the website name in it) and save it. This will clear the current thumbnail, allowing the new custom one to appear. After saving the generic thumbnail, open the thumbnail editor again and go through the options to find your newly made custom thumbnail! --!>
Honestly it's a bit overkill, but I find it's the most effective and simplest way to get those custom images in. You will need to host the images online (like imgur) due to security reasons browsers won't load local images on websites on the internet.
Thank you @kmielczarczyk for reaching out, it is greatly appreciated.
By any chance would the Opera team consider to bring back this function - that was really useful btw - so we can have a more enjoyable customization of Opera ?