I would suggest two things:
First buy the DVD and watch it, here it is on Amazon used for about $6:
Strickly Ballroom
It will make her happy that you went to the trouble to find her favorite movie.
I think it impossible to learn ballroom dancing from pictures or even an instructional video. A man has to lead in ballroom dancing and that comes from practicing with a partner.
So my second suggestion is to find a local teacher of ballroom or tango dancing and sign youself and your girlfriend up for a few lessons as a gift. Many colleges offer dance courses in their adult ed. section or check your local yellow pages under dance instruction.
You can buy a big heart shaped box and put in the video along with the class registration to present to her.