Image background color in menu

Hi there,

I've been searching through the forums and found a lot of topics how to create nice menuitems and how to add a picture to it (bmp or jpeg) but the only thing I didn't found is how to make the background transparant?

I also searched the helpfiles and developer guides, but didn't find a answer... Is there a specific color that I can use as a background color so that SBO (2004) identify that color as a transparant?

Thanks in advance...