This is the expected behavior of the RadRibbonDropDownButton, since its design follows the RibbonBar guidance specification.
However, I would like to point out a few things.
The SmallImage/LargeImage and Text properties are exposed to facilitate your efforts when sizing the RadRibbonDropDownButton through the Size property. By design when the Size property is set to "Large", the Text is always placed under the LargeImage and the DropDownButtonPosition is bottom. When the Size is "Small" only the SmallImage is displayed.
The Content property, on the other hand, is exposed to allow further customization of the RadRibbonDropDownButton content. Therefore, in your case you can define a StackPanel containing an Image and a TextBlock as a RadRibbonDropDownButton.Content. Then, if no Size is set, you will be able to control the DropDownButtonPosition.
I hope this helps you. Feel free to ask if you need more info.
All the best,
the Telerik team
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