Thank you for writing.
You have experienced the odd behavior because we do not support *.ico files in our ImagePrimitive responsible for showing images. This is true for the default Microsoft controls as well. We do not have any plans for to extend the range of supported formats, as this functionality actually comes from the .NET Framework API. You can easily use icons, however, by using the following pattern:
Icon ic =
Image img = ic.ToBitmap();
button1.Image = img;
Let me know if you have any other questions.
the Telerik team
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