In this study, a thermal neutron beam suitable for neutron radiography (NR) was designed based on the thermal column of the Tehran Research Reactor (TRR). The existing air-filled channel inside the graphite thermal column was utilized to implement a dedicated beamline consisting of a gamma filter slab, a boron carbide thermal neutron absorber with a central aperture, and a conical collimator. A comprehensive parametric optimization was performed using the MCNPX Monte Carlo code. A total of 144 configurations were evaluated by varying the gamma filter material, aperture thickness, aperture radius and the distance between the aperture and image position. Bismuth demonstrated superior performance compared with lead due to its lower neutron absorption and effective gamma attenuation. The optimized configuration, employing 5 cm of Bi filter and a 5 cm B₄C aperture with a 2 cm radius, achieved a thermal neutron flux of 1.0×10⁶ n·cm^-2·s^-1 at L/D = 114 under full-core simulation conditions at reactor full power. The neutron-to-gamma ratio and fast neutron suppression were significantly improved, while the gamma dose rate was substantially reduced compared with the existing E-beam tube NR facility at TRR. A secondary surface-source methodology was implemented to accelerate the parametric study and was subsequently validated against full-core simulations. Although the simplified model overestimated absolute flux values, it accurately reproduced relative performance trends, confirming its suitability for design optimization. The results demonstrate that the TRR thermal column can provide an efficient and high-quality neutron beam for advanced NR applications with enhanced beam purity and radiation safety.