Make Background transparency

[DaPoeschl]

Hello Guys,

I just started working with Gimp and it's pretty easy - so far. But i got a small Rpobnelm: I Have a layour with only black strips (about 2%) and a white Background. But this Background should be transparency, not white.
According to the Manual, i used the "Color to Alpha" Tool (I don't know, how is called in english. I hope, u understand). So i grapped the color, that's just "ffffff", got the Transaprency Threshold to 0,000 and the opacity threshold to 1,000. But it doesn't work.
What can i do about that?

Have a nice day,
Daniel

[Blighty]

The layer must have an alpha channel
Layer > Transparency > Add Alpha Channel
or right click on the layer in the layers dialogue and select Add Alpha Channel
(If Add Alpha Channel is grayed out there already is an alpha channel)

Also, the image must be RGB
Image > Mode > RGB