nope, doesn't work that way despite what your fantasy thoughts have you believe about how things work
a trip to cannes costs the production company/studios a lot of money per person (flights, hotel, food, party, etc.)... they aren't wasting it on "eye candy" because that doesn't matter and sophie isn't a big enough deal to get them more attention
either her role grows thus warranting the inclusion or her agency paid for it to get her name/image out there with hopes it'll pay off in the future just like they did for the recent paparazzi photos... a pap didn't just randomly spot her the other day grabbing a smoothie while wearing a tank top showing so much boobage, her people called a pap up and told them where she'd be and when and to go shoot her (heck, they may have paid the pap to do it)