Switching to the Overview tab for the first time will show a yellow banner which states that additional files are necessary to display Remote Desktop live thumbnails:
Clicking on Download now? will initiate the download of the required EasyHook DLLs.
Download the files manually
In case the download doesn't work, you can download the files manually:
Place those files in %appdata%\code4ward and restart Royal TS.
In case the files weren't downloaded correctly you can force Royal TS to re-download those files by using View -> Options -> Troubleshooting -> Delete EasyHook DLLs
Alternatively you can just delete the EasyHook32.dll and EasyHook64.dll in %appdata%\code4ward and download the files manually as mentioned above.