Use a 2D editor like Gimp that supports alpha and save the images as PNG files.
Edit:
Looking at your other post you want a alpha mask shader. To do this what you need is a shader that just mixes between the image alpha and the alpha of the sprite.
When you say “group of sprites” can you give an example of what you are trying to do?
Thx first. “group of sprites” means that “the sprite” is combined by several Sprite.
Structure in godot will be(project “SpriteMask.zip” come from github link above ):
Main
----TextureRect
--------child1
--------child2
--------child3
--------child4
----HSlider
the child1,2,3,4 is Sprite. And I want these to be cut off along with TextureRect.
The shader only effects the TextureRect.