Individual boot deals with say Nike are separate in that they are between the players and the sponsor and have an agreed value.
Image rights are in effect an agreement between the club who agree to pay a sum ( it used to be a set % but it’s now a quantified and independently validated number on a player by player basis) of a player’s overall pay in effect by way of compensation allowing the club to use the players image for instance in club commercial activities.
The principal of these payments are completely legit .
HMRC wanted to stop them in the day ( or Inland Revenue as it was then )but the lost a trial case against Arsenal and certain players but football being football the clubs, the player, the agents pushed it too far and the % of players wages that clubs were paying into companies was stupid some times 90% hence why The FA became involved and they and HMRC agreed a framework which remains in place to this day but….
When a player signs a contract all the financial details have to be listed on the contract and that is lodged with the PL and the FA and that includes the image rights payment
The rules in this country are clear that all of player’s remuneration per the contract have to be be made by the club.They can not be made by any other entity
It gets even more complex and something that’s a live topic in that HMRC are actively looking at payments in respect of image rights.They, HMRC , have no issue if they are paid in line with the terms of that Arsenal ruling because that tribunal gave the green light because these payments were paid into UK based companies who in turn will pay CT , if appropriate VAT , etc
The problem now is that if the payment goes to an off shore company, which many players have set up then it’s deemed by HMRC to be tax evasion. Most if not all PL and indeed Championship clubs are or were under investigation. My club , Chelsea, it seems have settled with HMRC the issue was disclosed during Bohleys due diligence and a vast sum ( tens of millions) was diverted from the sale proceeds to clear this matter up.