The island growth of ultra-thin metal films of Au, Cu and Pt prepared by ion beam sputtering on an amorphous carbon substrate has been analysed using digital image processing methods. The transmission electron microscopy pictures revealing topological evolution of the metallic films with an increase in the film equivalent thickness, have been fed into the computer and processed to determine the area coverage of the discrete metallic particles for various thicknesses. The average particle size has also been measured at a small area coverage. The behaviour has been analysed using a nucleation and growth model of metallic films. It is found that the power exponent alpha of the function f = At(alpha), where f is the fractional substrate area coverage and t the equivalent film thickness, for Pt is close to unity whilst those for Au and Cu are close to 0.64 and 0.71 respectively. The results are considerably more accurate than those published earlier without using such image processing methods, hence more reliable details of the island growth of metallic films are revealed. It is also found that at a small area coverage the average particle size of Au and Cu is proportional to the equivalent thickness. These findings, based on the theoretical model, give strong confirmation of a two-dimensional growth mechanism for Pt and three-dimensional ones with cluster (island) dissociation up to a critical cluster size of m atoms for Au and Cu. It is indicated that the dissociation value m for Cu is greater than that of Au.