[ Help ] Alpha in EditorGUI.DrawPreviewTexture

Hello,

in a EditorWindow i am not able to set the alpha transparency in a texture with:

EditorGUI.DrawPreviewTexture
I checked the Alpha texture in Photoshop and i saved it both in .png and .psd
I enabled the "Alpha is transparency" in the inspector, tried to enable the texture as GUI, but i have this result ( white in the red buttons should be transparent ):



Any suggestion?

Thank you.