Well, he didn't sue the Chinese channel owned by Suning of him promoting the Inter-Napoli game, did he? That's also part of the "image rights" issue. They did remove it, but there's no evidence to suggest that Messi lawyered up for that to be removed
Also, owning your image rights doesn't mean that no one else can benefit from having you advertising. Why do you think he still has sponsors? These image rights that are speicfied on contracts are entirely related to "Player as a part of the Team". Not exclusively the image rights of the person, but under his capacity of a part of the roster.
So Messi's salary is something like 25m net, he gets like another 25m on loyalties and a figure of around 10m on image rights. So effectively Barcelona is paying Messi this amount so that he can promote Barcelona, not the other way around. If he owned 50%, then they'd pay him 5m and the other 5m they'd keep for example. Image rights isn't that big of a deal. It can be a huge number (and it is) but in this specific case all that matter is the base salary and the latest trend is the loyalties. I think he also has 15m bonuses in case they win the league, he becomes top scorer in either of the two big competitions, they win the CL, he wins the World Cup, Ballon d'Or etc.
If Suning for example becomes Messi's sponsor, it won't affect his "image rights" deal with Inter. Teams do not sign sponsorship deals with their players, hence why "image rights" are necessary for both parties. Usually teams milk their players, but when they are bigger stars and own most or all of their image rights, they get well paid and essentially all it does is not allow clubs to parade them around with little economic benefit for them. If we want to make a couple of videos of him, we're not going to pay him for example. But if a club sponsor wants some players and he's one of them, then we'll end up giving him a bigger slice of the action than if he hadn't owned 100%. That's also negotiable every time, but there's no point in having an argument with the guy over this one. It's nothing in the grand scheme of things.