An NBA league source says the Boston Celtics ESPN first reported the news Tuesday evening. The league’s collective bargaining agreement allows teams to have an ad measuring 2.5 inches by 2.5 inches on their jerseys, beginning next season. The Celtics now join the 76ers and Kings as the three teams who have secured sponsorship deals. General Electric Co. will pay more than $7 million a year under an agreement with the Boston Celtics to put the company’s iconic round logo on the basketball team’s uniforms, according to a person familiar with the deal who asked not to be identified.