This is how I make them using Photoshop..Not sure if you have Photoshop or not.... But here goes
1. In Photoshop, Click on File, Open and navigate to your BMP File (We will use a Pine Tree just as an example) Click open
2. Now click on Image, Mode, then Indexed Color ( to Change it from RGB color)
3. Now Click on Image, Adjustments and then Replace Color
4. In the window that pops open you will have a small pic of your tree, click anywhere on the background behind it and then in the three sliders below the pic drag the arrows on all three to the left hand side. This will change the background to black (click OK) Make sure that Fuzziness slider at the top is on zero.
5. Now Click on Image, Mode and Grayscale a small box will open asking for you to discard color information just click OK
6. Now click on Image, Adjustments and Brightness/Contrast. In the window that pops up grab the top slider and drag to the left until you can barely see the difference in the background and the tree itself. Click Ok.
7. Now click on Image, Adjustments and the Replace color again. In the window that pops open click on the background of the tree and drag the Lightness slider all the way to the right to make the background white. Then click on Filter and Blur a few times.
8. Now click on Image, Adjustments and then Invert.
9.F inally click on Image, Mode and change back to Indexed color and save the file. It is ready to use.
Hope this helps in some way
Terry