Im wondering if its ok to use the username in the title or the real name of the photographer? For example in my case, is it ok to name the picture a picture by Mundi or a picture by Guðmundur Hauksson. I can´t find it in the rules that its not allowed.
Whether or not it is technically illegal (probable), I can say it would probably not be well-received by the voters. With the exception of self-portraits, it is understood that the entries are supposed to be anonymous.
Although, if you're talking about another DPC user, then it's fine... Just do be sure that it's in good taste, I have no idea what would happen if you broke the forum rules with an image title in a challenge........