I think I should start by saying that I am a UI Designer.
I am having a really hard time trying to convert 8 png's (same icon in 8 different resolutiona) into one single .ico icon for a Windows app. The png's resolutions are the fallowing x16, x20, x24, x32, x40, x48, x64, x80.
I have tried every convertor online there is and even some obscure softwares and nothing...
Since you posted this to Stackoverflow I'm going to start by linking you to the blog series about the icon format history, specifically the PNG entry.
The best choice for working with icons is special purpose icon editors, they give you full control of PNG compression and other details. A decent free editor you can try is IcoFX.