Image rights are the commercial value of a player's name, face and likeness, licensed to the club separately from playing wages. The club uses them in shirt sales, sponsorship and advertising. Because they are paid to a company rather than as salary, tax authorities scrutinise how they are structured.
For most squad players the arrangement is small and routine. For a player whose face sells shirts and appears in a sponsor's campaign, it can be a substantial second income stream and a real part of what makes one offer better than another.
The tax question is why it draws attention. Payments for a licence to use a likeness are treated differently from wages in many countries, and revenue authorities in several major footballing nations have pursued cases where they judged the split between the two to be artificial.
It also affects transfers directly. A player who has already sold their image rights to a third party, or who holds them personally on terms a club will not accept, is harder to sign than the wage discussion alone suggests, and negotiations can stall there long after the fee is settled.