Yeah I myself am another example of what you're saying so I def believe it . I wouldn't say I was blonde but I had light brownish hair and it's apparent in many pictures but that was when I was 5-10 now i have PURe black hair! Very weird but anyways that picture looks a bit more blonde than I was however maybe its sun I dont know.
I think that Ronaldinho pic is actually of Ronaldo Nazario, based on skin tone (clearly lighter than Ronaldinho de Assís) and on the fact that Ronaldo was actually dubbed Ronaldinho in his early days in the Brazil National Team, which shirt he is wearing. That's why I think that's actually Ronaldo - plus, it resembles more to him than to Ronaldinho.