Image rights are up to 20% of a players total income (limited to that amount by HMRC in the UK, in other leagues it can be much higher).
It's not 3rd party ownership, it's basically a licensing deal where Juventus pays a portion of Dybala's income to a company he has licensed to manage his image (and is currently in dispute with).
In the UK, HRMC only allows image rights to be licensed to other UK registered firms, and payment to be capped at 20% of total wages. Effectively this means that Dybala has to buy out his current licensing contract and then sign a new one with a UK registered firm.
The licensed firm doesn't have to agree to the buyout, obviously