First of all, what Design Programme are you using? Paint.net? Gimp? MsPaint.
If you're using Paint.net, use the magic wand and make all the parts you don't want transparents, it should look checkered if you've done it right. (White and grey)
But lower the tolerance so not everything is transparent.
MSPaint, pure black works or there is that magenta color. Sorry, I'm not sure what color it is.
Gimp. No idea, don't use it.