I am glad to hear that you have managed to resolve this. I would only like to add that a larger image may look like it isn't there if there is enough white space in its top left corner. It is applied as a background image for an anchor element and thus the normal CSS rules apply for it.
You also need to consider transparency, as a transparent background image would also appear as non-existent - when you set the IconUrl property the RadWindow adds the following rule for the anchor element background:
Also, if you set the IconUrl property you do not need to add the CSS override, this will be done by the RadWIndow.
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