When creating a site a favicon is pretty much a must these days and it has always bugged me that creating a favicon isn’t straight forward. For one thing different browsers support different standards of favicon, most modern browsers support png images but older browsers only support bmp images. Generally speaking this isn’t an issue and the .ico format allows for this as essentially it is simply a container format which contain several images, a browser in general will choose the most appropriate image for the correct right situation, and you can even specify which icon to use using a meta tag.
Creating an .ico file however which contains several files can require a specialist piece of software which usually isn’t free. You can of course use an online service to do this but I have had mixed results in creating them with this, especially when you want to provide many alternatives.
I have stumbled across a different method. You can use plain old ImageMagick to create them. Simply use the the following command:
convert image1.png image2.png image3.bmp favicon.ico
And that’s it, a favicon is created containing all of the images.