What size should the source be?

At least 512 x 512 px, or an SVG. Smaller sources will look soft in the larger icons.

Are my images uploaded?

No. Images are decoded, processed and re-encoded by your browser on your own device. They are never sent to a server, and closing the tab discards them.

Favicon Generator - Free Online Tool

JavaScript is turned off. InspireTools processes files in your browser, so the tools need JavaScript to run.

Create a complete favicon set: ICO, PNG sizes, Apple touch icon and manifest.