If you have a batch image conversion to do, ImageMagick is your friend. It provides commandline tools for manipulating images.
I recently had to convert about 200 .svg images to .png images, while maintaining the alpha channel. Rather than doing this by hand, it can be done in a single command, using mogrify, one of the ImageMagick utilities.
mogrify -background none -format png *.svg
Have a look here for a list of the other arguments that can be passed to mogrify.