How I can apply this texture on Material Manager?

I am having trouble creating textures with a transparent background. I have tried searching for a solution to this problem, but I haven’t been successful so far. I am considering using decals, but I’m unsure how to proceed because I have multiple textures that need to be applied.

The site I am using for this textures is called 3D TEXTURES

The background Transparency doesnt apply
image

You could use a program similar to Photoshop to change the opacity of the image. This MIGHT work, but it possibly could not.