Usually for the tree you will use only one texture, that already has got the opacity map built in, in the alpha channel, that's the highest 8 Bit of a 32 Bit TGA for example.
You may also load any 24 Bit RGB texture with the alpha flag (2), this will automaticly create an opacity map during loading, making the dark pixels more transparent and the bright pixels more opaque. Usually this is not what you want, so I'd suggest to use a texture with an alpha channel. You may also use PNG, but you have to use transparency of "unpainted areas", not an additional channel as you would do it for TGA. This is done easily with Photoshop, and as a substitute you may use The Gimp that allows similar actions and is available for free.
You may also load any 24 Bit RGB texture with the alpha flag (2), this will automaticly create an opacity map during loading, making the dark pixels more transparent and the bright pixels more opaque. Usually this is not what you want, so I'd suggest to use a texture with an alpha channel. You may also use PNG, but you have to use transparency of "unpainted areas", not an additional channel as you would do it for TGA. This is done easily with Photoshop, and as a substitute you may use The Gimp that allows similar actions and is available for free.