Polaroid - Impossible to save with white background as jpg

[sabater_wb]

Hi guys.
I'm trying to run this code:
"convert -caption "Faerie Dragon" dragon.jpg -gravity center -background black +polaroid anno_polaroid.png"

The problem, is that when I save it as jpg, its background is saved in black.
I tried even save it as png, and after convert it to jpg, but the result is the same.

How can I save it as jpg and with a white background?

thank you.

[anthony]

Polaroid generates a transparent background, JPEG can not save transparency, so it comes out black.

For JPEG -flatten the image first!!!!!

See IM Examples, Common File Formats, JPEG
JPEG transparency - NOT
http://www.imagemagick.org/Usage/formats/#jpg_trans