Black and White JPG to Transparent PNG
To create an image with a transparent background that's black and white, the process is simple. Open your image. Select Layer>Transparency>Add Alpha Channel.
Now select Layer>Transparency>Color to Alpha. Since you are removing white, you don't need to change the color. Note: if this was a color image, any color containing white, the white will be removed, giving the colored areas an opaque effect, or in the digital scrapbooking world - vellum quality.
If you want this new image to be a brush, then go to Image>Mode>Grayscale. In 2.2 you go to Script-Fu>Selection>To Brush. Name the brush and the script will create it and add it to your currently selected brush. In 2.4 go to Edit>Copy>Paste As Brush. Name it and it'll pop up in your brushes tool box. Making it grayscale will allow the brush to take on the currently selected color in your palette. More about brushes were posted elsewhere.
Be sure to save your finished product as name.png - if you save as a jpg you'll get the white background again.