I use the method described here:http://ftm.custhelp.com/app/answers/detail/a_id/5160
Summary: Go to People workspace, Family tab. Right-click on the existing primary photo in the summary panel at the top right. Left-click on "Link to existing photo". Select the photo you want to use and click "OK".
Sadly there's no easy way that I've found to just take one of the existing linked pictures and make it the primary - this is very easy on Ancestry.com but on FTM (using the above method) you have to sort through every picture in your media gallery - rather than being able to select from the few photos already associated with that person. If anyone else has found a simpler method I'd love to hear of it. What I always end up doing is selecting the primary photo on Ancestry.com and allowing TreeSync to make the change for me in FTM.