Gaara's code should work, you can just change the opacity and different brushes to customize the look.
But if you already have stuff made and all that, you can use this code to make anything partially transparent, without having to change the image:
material preset_(PresetNumberHere)_(BodyPartHere)Mat(Left(L)OrRight(R)Here)
{
technique
{
pass
{
alpha_rejection greater_equal 128
texture_unit
{
texture (TextureNameAndFormatHere)
}
}
}
}