With your video preview window open and set to "Best (full)", go to the frame you want to make your jpeg from and click on the floppy disc icon at the top of your video preview window. Make sure you have it set to "Best (full)" to get the highest resolution image.
I do one additional step. I open the captured picture in Photoshop, scale it to half hight, then export it to Photozoom Pro and expand it back to full height using one of the spline options. If I want a bigger picture I resize to that size now making sure that the height number is twice the width. This gives me the highest quality deinterlaced still possible.
By the way, I've also tried this with downloaded footage from the FX-1 and the results are spectacular.