PEAR Image_Transform

Can anyone help me write a code using this class to scale JPEG
pictures? I have n o idea how to write a function that would look like
this:

function scale($picture, $name, $size)

where:
$picture is the base JPEG file
$path is the the name of the new file
$size is the new size of the longer length

f.e.

scale("pic.jpeg", "pic_new.jpeg", 300)
[the pic.jpeg is f.e. 600x300)

i will get a new pic called pic_new.jpeg with dimiensions 300x150

Can anyone help me with that? There are so little examples on PEAR
site..If you prefer using GD itself, that is fine. Whatever will work
will be great.

Thanks for help!
Jakub Moskal