Is it posible to set/change the default "save to web" file format

Sometimes I have a sliced up file and I would any and all new slices to default to a png, but for some reason the default on the file is jpeg. It seems random from my point of view as it always seems to change. Is there a settings for this?

I would like to set it so thatt all new slices are set to save in a particular format (ie gif-128-dithered).