Note that this control by default browses all files not just images, but if you get the very latest code from the repository I just added a new property on the control to make it configurable to filter file types. So if you just want images you can set the browser type property like this:
<portal:FileBrowserTextBoxExtender id="fileBrowser" runat="server" BrowserType="image" />
valid options are file, image, and media
from code you must set the TextBoxClientId property to the textbox where it should set the file path like this:
fileBrowser.TextBoxClientId = txtUrl.ClientID;
where txtUrl is the server side id of a TextBox control