Two methods of image analysis of cross-sections through two-phase porous media are compared. Linear filtering uses the recursive (IIR) Gaussian filter with the same vertical and horizontal blur radii. The images are also treated by removing small clusters of pixels in the void and solid phases. After linear filtering and before removing small clusters, the grey-level threshold is determined in order to partition all grey pixels into pore and solid pixels. Linear filtering reduces the specific surface very efficiently creating ``smooth'' pore walls and leaves small clusters of pixels in the images. Removing preserves ``rough'' pore walls and removes simultaneously small clusters from both the phases