For resize & background. Use the following, and note that you'll need to calculate the 300% yourself.
from wand.image import Image
from wand.color import Color
with Image(filename="pic.png") as img:
# -resize 300%
scaler = 3
img.resize(img.width * scaler, img.height * scaler)
# -background white
img.background_color = Color("white")
img.save(filename="pic2.png")
Unfortunately the c method MagickMergeImageLayers has yet to be implemented. You should author an enhancement request with the development team.
Update
If you want to remove the transparency, just disable the alpha channel
from wand.image import Image
with Image(filename="pic.png") as img:
# Remove alpha
img.alpha_channel = False
img.save(filename="pic2.png")
Another way
It might just be easier to create a new image with the same dimensions as the first, and just composite the source image over the new one.
from wand.image import Image
from wand.color import Color
with Image(filename="pic.png") as img:
with Image(width=img.width, height=img.height, background=Color("white")) as bg:
bg.composite(img,0,0)
# -resize 300%
scaler = 3
bg.resize(img.width * scaler, img.height * scaler)
bg.save(filename="pic2.png")