Lastig. Het zou kunnen gaan om het vermelden/ondertekenen van de naam de speler op de genoemde foto, maar bijvoorbeeld ook om een sponsorcontract of het promoten van sportkleding of iets dergelijks. Zie de onderstaande definitie van Merriam-Webster.

http://www.m-w.com/dictionary/endorse.
1 a : to write on the back of; especially : to sign one's name as payee on the back of (a check) in order to obtain the cash or credit represented on the face b : to inscribe (one's signature) on a check, bill, or note c : to inscribe (as an official document) with a title or memorandum d : to make over to another (the value represented in a check, bill, or note) by inscribing one's name on the document e : to acknowledge receipt of (a sum specified) by one's signature on a document
2 a : to approve openly <endorse an idea>; especially : to express support or approval of publicly and definitely <endorse a mayoral candidate> b : to recommend (as a product or service) usually for financial compensation <shoes endorsed by a pro basketball player>