Slope failures have caused significant human casualties and economic losses worldwide, with impacts reaching billions of dollars annually. These failures underscore the critical need for effective slope stability analyses. In particular, the mining industry faces this risk due to the accumulation of waste in tailing dams, which have a history of catastrophic failures. This study introduces a novel approach to analyze slope displacement fields and failure surfaces using the Digital Image Displacement (DID) method for an optical flow analysis, combined with a MATLAB code for data correction, numerical verification through Finite Element Method (FEM), and Limit Equilibrium Method (LEM) analyses. This novel methodology in geotechnical applications provides high, pixel-level sensitivity, thus enabling the detection of small displacement fields over short time intervals and spatially localized motions necessary to track the initial development of progressive failure. Forty-one experiments were conducted on untreated and geosynthetic-reinforced sandy slopes, thereby using more than 700 high-resolution digital images to investigate failure shapes and displacement fields. The results revealed a logarithmic spiral failure shape for both untreated and reinforced slopes, with reinforced slopes exhibiting deeper and more pronounced rupture surfaces. Geosynthetic reinforcement significantly improved the slope stability, which resulted in the Factor of Safety (FS) increase for 30° slopes from 1.2 to over 3 and increasing load-bearing capacity (up to 1700N) while reducing displacements. Additionally, the study emphasized the critical influence of slope geometry and compaction on the overall stability, thereby identifying closer reinforcement spacing (e.g., within one-third slope height) as the most effective design configuration to maximize stabilization. These findings offer valuable insights into the slope behavior, improve the precision of displacement measurements, and advance geotechnical engineering practices for enhanced risk mitigation and slope stability analyses.