You can automate this conversion by using the freeware XnView (http://www.xnview.com/en/xnview.html) with NConvert (http://www.xnview.com/en/download_nc.html)
The DDS files are supported.
You can simply download and use the portable version (zip) : XnView v1.98.5 Complete version (Some Plugins/Addons & NConvert, ShellEx, Media Detector) (http://download3.xnview.com/XnView-win-full.zip)
Then go in
Tools >
Batch Processingvisitors can't see pics , please
register or
loginTo quickly load settings, create an empty text file and copy and paste this :
resize( 1 lanczos 50% 50% -1 1 0 )
settings( 0 0 1 0 0 1 1 )
output( dds )
and save as Resize 50 percent.xbs
It took around 2 min to convert 344 dds files.
Enjoy