Lynde was sentenced in 2010 after pleading guilty in federal court in Aberdeen to one count of felony possession of child pornography.
Prosecutors say Lynde was charged with knowingly possessing graphic computer image files of child pornography on a USB storage device and a computer.
At the time of his arrest, Lynde was serving as the interim chairman of the DSU history department.
On Thursday, the 5th U.S. Circuit Court of Appeals dismissed Lynde's challenge to his sentence as too harsh. The 5th Circuit says the penalty followed federal sentencing guidelines.