Webp
https://en.wikipedia.org/wiki/WebP
WebP is an image format employing both lossy and lossless compression. It is currently developed by Google. It is a sister project to the WebM multimedia container format. It is an open source alternative to JPG and PNG.

WebP is a modern image format that provides superior lossless and lossy compression for images on the web. Using WebP, webmasters and web developers can create smaller, richer images that make the web faster. WebP lossless images are 26% smaller in size compared to PNGs. WebP lossy images are 25-34% smaller than comparable JPEG images at equivalent SSIM quality index. Lossless WebP supports transparency (also known as alpha channel) at a cost of just 22% additional bytes. For cases when lossy RGB compression is acceptable, lossy WebP also supports transparency, typically providing 3× smaller file sizes compared to PNG.

If you are running Chrome, you get webp images exclusively on those websites, while Firefox or Internet Explorer users get the standard image formats instead. The websites in question check the browser you are connecting with and either deliver webp if supported, or fall back to jpg or png if it is not...programs like XnConvert support the webp format so that you can convert the downloaded images into another image format.There are numerous online converters available as well for that.

XnConvert is a powerful and free cross-platform batch image processor, allowing you to combine over 80 actions. Compatible with 
500 formats