Thank you for writing. The RadComboBox was not designed to contain images in its text box. Trying to put an image there will be too cumbersome to implement and will mix up the layout of the RadComboBox. You can simply add an image right before your combo or somewhere else that you see fit.
I am sorry if this causes you an inconvenience but this was not part of the original design. Please write back if you have other questions.
the Telerik team
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