Do you have Gimp? Do you wish you could batch process an arbitrary number of textures? Well now you can. What you need is Bimp (Batch Image Manipulation for Gimp). It's not perfect but it has just saved me a whole bunch of time.
TR
Another free program that lets you do this, which I use, is Irfanview.
New update for gimp. This is a minor update from 2.8.16 to 2.8.18, released just 5 days ago. The portable version on portable apps has been updated as well.
While I'm at it, Bimp allows you to batch process images using (at least in theory, I had no need to actually try it) any single gimp command.
TR