I simply want to have an .ico file that has multiple sizes of the icon image contained within it. I'd like it for use in a cross-platform desktop application (so that, e.g. on Windows, the 16x16 size is used for the app's top bar but a 32x32 size version is used when the various open apps are shown when using Alt-Tab). Once I have that .ico file, I know how to use it within my widget toolkit to get this effect, but I don't know how to get it.
What process should I use to make such a file?
This can be done for free using GIMP.
It uses the ability of GIMP to have each layer a different size.
I created the following layers sized correctly.
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