I uploaded a PNG with transparency to this favicon generator:
The 32x32 and 48x48 Windows desktop previews displayed on the site look like they have very bad transparency (1-bit?). I don't have a Windows or MacOS machine to test them. How can I find out if there is a problem with the transparency of the 32x32 and 48x48 images in my favicon.ico?
Apart from testing them in the relevant browsers / systems, you could show them to us and we can take a look. Perhaps make a test page with all of the icons there, and we can see how they look on each system.