Texture smoothing

Someone knows how to smooth a texture? because im reamking that trueno mod, and i was trying to do the
Tofu door, but the image that i've choosen was a 4000x(something very lower than that)pixels, and i rescaled it to fit the 1024x1024 pixels texture, but it has some "squared" bords, how do i smooth that?Also im using Paint.net
Here is whats happening:

 

Get a higher resolution texture, it will look like crap otherwise.

 

Make sure your scale quality settings are set to high before you scale down the texture.