Favicons are generally 16x16, but these days we can multi-resolution favicons also. To create a multi-resolution favicon, first, you need to create the highest-resolution version of the favicon, with an image of 256 × 256 px. Reduce the size of multiple resolutions and export, 16x16, 32x32.
Open GIMP and from the File menu, open the 32x32 version of the Favicon. After that rightclick on the image and Open the other Favicon icons as layers,
After that Export the image as Favicon, with extension .ico and add it to the website using the following code −
<link rel="icon" href="/favicon.ico" sizes="16x16 32x32" type="image/png">