According to this page it should be possible to use the option -jpeg
to generate a jpeg image as result.
If you can't you should get a newer version maybe, if you are allowed to.
If not the only solution is to convert it afterwards. But in any way, if you are having the feeling that the JPEG conversion is too slow, I'd give you the advise to use libjpeg-turbo. If you replace the standard libjpeg library you should get a performance boost of the factor 2 at least.