<p>I'm using Visual Studio icon library (VS2008ImageLibrary), there are some BMP files with a pink background. How can I make the pink background become transparent? What software can I use to do this? Any free one? </p>

<p>Thanks</p>

I used IrfanView's batch conversion tool. It's still some work, because you have to click the pink area on every single icon instead of just specifying pink as transparent color, but it worked. It would be easy to write a simple conversion tool using GDI+ though, which I considered as well.
Note that even though you can use them then as transparent images, many of them unfortunately still have ugly edges when you render them on a dark background or use them as overlay. If you just want very few of them, consider tweaking them individually if you think you'll not always have a bright background.