The pictures, showing Senegal international Diouf being abused during a game at The Den on 18 November, appeared on Sky Sports News.
"Like all Millwall fans and football fans generally, we're appalled by the footage we've seen," said Lions chief executive Andy Ambler.
"Clearly that behaviour is unacceptable in any football ground in the country."
Ambler added: "Sadly racism exists in our society and certain individuals want to bring it into football grounds. It's down to us in football to do more.
"That day against Leeds obviously there was an allegation of abuse made online initially. We investigated it straight away.
"We interviewed stewards, police, players, we looked at the TV footage too as it was live on Sky.
"After speaking to the player [Diouf] who had the alleged abuse at him during the day, he said that he heard no abuse so that was basically at that point the end of the investigation - although we're going to open it again now."