Uefa took disciplinary proceedings against Celtic over the images of IRA hunger striker Bobby Sands and Scots freedom fighter William Wallace, which was displayed ahead of their home defeat by AC Milan.
The governing body yesterday confirmed it had fined the Parkhead club 50,000 Euros over the incident.
Club bosses said it was "extremely disappointing" that Celtic had been hit with "another sizeable penalty following the actions of a small minority".
In a statement, Celtic said: "Regrettably, due to previous charges being brought against the club, again following the actions of a small minority, the fines imposed by Uefa are increasing in scale.
"It goes without saying that such actions must stop now, before the club receives a competitive sanction or one which would affect our supporters attending European matches.
"We are sure our fans will come together, unite with the club and support the team with the commitment and passion we are famous for."
Celtic manager Neil Lennon previously revealed that his "heart sank" when he saw the Green Brigade banner prior to the AC Milan match.
Last month, Ajax were fined €20,000 for crowd disturbances and the throwing of objects during their Champions League game in Glasgow against Celtic on October 22.
The Dutch club was also fined £21,000 by Uefa after fans displayed an offensive banner at their home game against Celtic.
The banner carried a sectarian message and appeared area of the ground used by the club's 'F-Side' hooligan group.