Hello Sergio,

Thank you for reaching out and I am happy to help!
If you are referring to the webp converter in the W3 Total Cache, you have to initiate the optimization manually. In Performance>Extensions, under the Webp converter settings, you can bulk convert all images, or you can manually convert the images in the media library once the extension is activated.
As for the newly uploaded images, those will be auto-converted depending on the settings in the webp settings area:

Let me know if you have any other question
Thanks!