I used Paintbrush drawing an icon, if it is on a colored background, it will display a white block shown as below.
I prefer to change the icon background from white color to transparent color:
Do I need a special tool?
Personally, I use GIMP for all my graphics editing needs. It makes it trivial to make a png with transparency. If you do a lot of graphics, or even just playing around it is worth the download. Also, it's free.
You can actually do that using Paintbrush (Mac)
Select a foreground color - any color will do as our intention is to remove the background.
Next, drag the opacity slider from 100% to 0%. You should see your selected color turn into a grayish tone.
Now click the fill icon and click anywhere in the background.
You should see the background is now transparent.