I reviewed the icon editor IcoFx about a month ago which offered the function to create an icon from an image that would be loaded into the software. It offered some benefits like different sizes and a configurable color range which made it one of the best tools for that purpose.
Shell City mentioned the online script Bradicon today which is a stripped down version of the function in IcoFx. If IcoFx would be a commercial product then Bradicon could be its free version. It basically accepts an image and creates an icon from that image.
The icon will be provided in ico format as a download after the creation has finished. The icons that I tested were provided in 48×48 format only but it should not be a problem to resize them manually. The website contains no information about the supported image formats, I was able to upload png and jpg images without difficulties.
Bradicon is a nice alternative to IcoFx or similar products especially if you have to work in an environment where you are not allowed to download and install any additional software.
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This is a great tool, and very fun to work with in your spare time.