The incident comes at a tumultuous time for the department and an uproar over the Confederate flag after a racially motivated shooting at a historically black church in Charleston that left 9 dead.
Sgt. Shannon Dildine allegedly posted the photo several days ago and the image has since gained attention as it circulated on social media, according to ABC News Affiliate WCIV.
"Your posting in this manner led to you being publicly identified as a North Charleston Police officer and associated both you and the Department with an image that symbolizes hate and oppression to a significant portion of the citizens we are sworn to serve," Police Chief Eddie Driggers wrote today in a letter, obtained by ABC News, to Dildine.
The letter slammed Dildine for significantly undermining "your ability to improve trust and instill confidence when working with our citizens."
It also said that the move undermined the sergeant's ability to handle criminal cases in which minorities were involved "since defense counsel can reasonably be expected to use the photograph to call into question the [sic] your motivation in making the arrest..."
While Driggers says that "some say the flag emblem may have different meanings to different people," he writes that "your personal beliefs are irrelevant to the City's decision to terminate your employment.
"In light of current events posting an inflammatory photograph in a way that permitted it to become widely distributed shows a lack of reason or judgment that is unacceptable," the letter says.
According to the letter, Dildine has 10 days to appeal.
The North Charleston Police Department did not immediately respond to ABC News' request for comment.
Slager allegedly shot the victim as he was running away in an incident that was caught on video.
The incident also comes as the uproar about the Confederate flag has reached a fever pitch.
Aside from calls to remove it from the South Carolina statehouse in the wake of the Charleston church massacre, a number of major retailers have stopped selling it at their stores and online outlets.
The cause to remove the flag has also been taken up in other Southern states.