To gain insight into the origin of the colossal reduction of resistance in response to magnetic field in colossal magnetoresistance manganite, the magnetic field induced transition in ferromagnetic La0.7Ca0.3MnO3 was studied using a high-resolution magneto-optical imaging (MOI) technique. The MO images were captured in various magnetic fields over a wide temperature range for both highly dense samples with strong-link grain boundaries and porous samples with weak-link boundaries. Formation and evolution of magnetic domains as a function of field or temperature were clearly observed around and far below the Curie temperature TC=240 K. Ferromagnetic areas tend to grow to large sizes and finally join together at the expense of paramagnetic areas as the field increases or temperature decreases for strong-link samples. A sharp magnetoresistive transition is observed when the sample changes from a paramagnetic insulator to a metallic ferromagnetic phase in the vicinity of TC. In contrast, the porous samples showed magnetoresistance over a wide temperature range and exhibited a remarkable grain boundary related magnetization process in addition to magnetization within grains. A close correlation is found between the magnetization process observed by MOI and magnetoresistance measurements. Our MOI results indicate that the strong-link or weak-link grain boundaries are responsible, respectively, for magnetoresistance occurring either only in the vicinity of the ferromagnetic transition or over a very wide temperature range.