Keep transparency, lose the weight
PNG is the usual format for logos, icons, illustrations and product images with a transparent background. The downside is size: a large PNG, especially one that contains photographic detail, can slow down a web page. Converting to JPG would make it lighter, but JPG turns transparent areas white.
WEBP gives you both. It supports transparency like PNG and usually produces lighter files, so a product cut-out still floats over your store's background and a logo still sits cleanly in your site header.
Screenshots and illustrations for the web
Tutorials, documentation, blog posts and help pages often use lots of PNG screenshots. Converting them to WEBP before publishing can make a long article noticeably lighter to load. Keep the quality high for screenshots with small text, so the letters stay crisp.
For illustrations and graphics with large flat areas, try a medium quality and compare. Because you can see the result before downloading, it's easy to find the lowest setting that still looks right.
Check where the image is going
WEBP is made for the web and modern browsers show it without problems. Some older desktop programs, printers and email clients don't, and some platforms still only accept JPG or PNG. If you're sending the image to a person rather than publishing it, PNG or JPG may be the more practical choice.
If a PNG is going to stay a PNG, “Compress image” can still make it lighter by reducing it to a palette of up to 256 colors.
Private and 100% free
The conversion happens in your browser, on your own device. Your PNG files are never uploaded, never stored and never seen by anyone else. There's no sign-up, no email and no limit on how many images you convert.
If the original file carries EXIF data, such as a GPS location from a phone, it isn't copied into the WEBP.
A quick routine for web graphics
Before uploading graphics to a website, it helps to check three things. First, the dimensions: if a PNG is much wider than the space it will fill, shrink it with “Resize image”. Second, the format: convert it to WEBP here, keeping transparency. Third, the result: open the WEBP and compare it with the original at the size it will appear on the page. If it looks the same, you've saved weight without losing anything visible to your visitors.