Looking at the picture you have posted, I would slightly change my suggestion to quilting an oval around the embroidered picture, like a photo frame. You could do this as a single line, or a double or even triple line; you could even do a double line and then add an edging of half circles or half ovals reminiscent of a lace edging. It all depends on what you fancy, whether you are going to hand or machine quilt it, and how confident you are on your skills.
For the overall design of the quilting - again it is easier to work out once you can see a layout of the pieces to be quilted, even something as simple as a coloured pencil drawing. I have some ideas about how you might
be placing them, - 6 x embroidered + 6 x pieced =12 blocks, either a 3x4 or a 4x3, probably alternating, presumably with a border to contain the whole. If you then block out the ovals of the embroidered areas, you need to consider the negative spaces left. What might work could be twisted rope style borders that link up, separate and link up again with other strands. I am thinking of the picture of the Lod Mosaic floor that Bridget posted in 'Anything Else' http://www.thequiltshow.com/bb3/viewtopic.php?f=118&t=5283&start=15#p78341
or maybe a celtic knot work pattern would work?