As you may know RadBinaryImage itself does not have built-in uploading functionality. Therefore, if your requirements is to store smaller sized images, you can resize the them manually when they are uploaded into the database and then use the already resized image's data to populate the RadBinaryImage control.
Regarding your other suggestion, I will forward it to our development department for further consideration. Thus if this appears to be a common request and attainable enhancement, you may see it included in a future version of the product. In the meanwhile you may consider manually applying the watermark to the image data before it is passed to the control.
the Telerik team
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