For a simple image like this you could use the "Selective Colour Mask".
Place your decal, in the "Mask Image" tab, select the "Selective Colour Mask" option. With the colour picker tool choose the white part of your image in the preview window, NOT on the decal on your model.
That should do it.
Thanks, it works for this case, but I need edit decal first, because it removed all white area. I think Solidworks should do this job, but just couldn't find it how.
Sorry, I thought you wanted to remove all the white.
If you want to keep the triangle area with the white still in it, you will need to use an image mask or Decal image alpha channel. Image mask requires a second black & white image with the triangle you want to keep in white & the rest black (this can be the other way around as the mask can be inverted with a check box). Or you could edit the image, make the section you don't want transparent, save as a png & use the Decal image alpha channel option.
Kevin's method should work well but here's another possibility.
If you have a graphics program (photoshop or similar), you could set the white color to be "transparent" and save the decal as a .png format. When that png file is applied as a decal the transparent color should not be shown.