The IrfanView freeware, has Batch Conversion/Rename utility
It handles correctly bitmaps.

For PNGs, I used Jasc PSP8, GreyscaleInPlace.PspScript with Batch processing

>Hi Dorin,
>
>> The most elegant for me was a set of light gray images, built with batch processing. Looks great on disabled buttons.
>
>Thanks for the suggestion. Can you tell me what application you are using to batch create light gray images?
>
>Regards,
>Malcolm