I found a solution to this. There's a chrome extension called Transparent Standalone images. Now this is a chrome extension but I added it to my OperaGX too. If you dont know how to add chrome extensions in Opera browsers, you can google it and find some videos on how to do it. Adding this extension and restarting the browser made PNG images with transparent background have transparent background in preview finally, 
YAY!!! Link to the extension :
https://chrome.google.com/webstore/detail/transparent-standalone-im/inmelfailofaofnfdjdkljfchjhogbmh?hl=en