Sometimes it is necessary to add some functionality for the Gallery Manager.
It can be done by adding custom button(s) to the Manager's toolbar.
In this example you can see customized "Browse Image Gallery"
The custom button click just causes an alert with the selected image URL here, but you can modify the client-side functionality as you want.
To open this popup, click "Browse" button in the "Image properties" popup (insert or edit an Image
The source code of this example you can see in the downloaded Suite
See files: /HTMLEditor/cs_CustomImagesBrowser1.aspx(.cs), /HTMLEditor/vb_CustomImagesBrowser1.aspx(.vb),