If you ever tried to make Nagios Status-Map a little nicer by adding some dependencies and some pretty icons, you have probably came across some little collections/packs of icons at NagiosExchange.
But if you need a special one, you must do it by yourself.
And you probably noticed that you need images with .gd2 extension
Here is how to convert a PNG image into .gd2 (GD Graphics Library format)

Solution one:

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aptitude install netpbm

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pngtopnm example.png | pnmtopng -transparent =rgb:00/00/00 2>/dev/null | pngtogd2 /proc/self/fd/0 example.gd2 0 1

Solution two:

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aptitude install libgd-tools

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pngtogd2 phone.png phone.gd2 0