If you want to change this path multiple times, you will need to do it Client Side. There is KB article showing how to set the value of this property by passing the text to a text-box (which could be hidden), available on the following link: Using the Image and Document managers outside RadEditor.
For your convenience we modified the example from the above mentioned article, so it is showing a preselected image or file now, so you can extend it further to match the needs of your custom scenario.
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I'm using the example provided by Vessy.
One textbox (contain the path to the file), one button to open ImageManager via Dialog.
When opening the imagemanager while the path is empty, everything is fine : imagemanager open the root folder.
Uncaught TypeError: Cannot read property 'length' of null
And then impossible to select a new image.
This error doesn't appear with the document manager so it must be something with the imagemanager. I also noticed that the image is selected but the informations about the image (width, height, ...) remain empty.
(sorry my english isn't very good)