If you are using Google Chrome web browser then you might have noticed that by default if you save images from selected websites (mostly Google Product websites) then by default it saves images in WebP format.
As per Google, WebP format reduces the size of image by compressing it without loosing its quality. But this happens only on Google Chrome not in IE or Mozilla Firefox.
Actually when you visit a website, let say Google Play then that website first check your web browser and if its Chrome then it will offer WebP format else traditional PNG or JPG.
Most of image viewers support WebP formats and you can view them easily. If you downloads images very rarely then either you can convert them using any Image convertor or you can use Firefox or IE.
But if you want to use Google Chrome only and use to download lots of images which is not easy to convert all of them so you can use Save Image As PNG Google Chrome Extension.
This extension will adds a option in your context menu which helps you to download images in PNG format no matter if you are using Chrome.
Its not clear how this service works, it author just says that it will convert images on fly but he hasn’t mentioned any of service which it uses so its little unclear at this time but overall this extension works fine.