I'll give it a go. First, you may have a more recent flexi than my 7.6 on the mac. You may also want to sign up on SI's site as they have a couple of good tutorials on this.
1. select image
2. select "make transparent" under the bitmap menu and select white space
3. select "invert selection" under the bitmap menu
4. select "convert marquee to shape" under the bitmap menu
5. from this point you have a vector line around your graphic. Alter it to fit your needs, select it & the jpg and mask or simply convert the vector line to a contour line and send along. My preference is to keep the masking and the contours seperate parts.