Overlays with transparent parts?

I’m trying to make a door frame using an overlay, but the texture has a black area around the door. Is there an easy way to set this black area to transparent, or do I have to mess around with the U/V start/end numbers to clip it off?
Thanks for any help.

You have to use a texture that is made for being a decal should be like: decals/texturename in the texture browser.

you have to create a new alpha channel and define the black area as black in the alpha channel, so it will be transparent. White areas are fully visible, shades of gray are fading transparency