Emulsions of oil and water stabilized by adsorbed solid particles are known as solid-stabilized emulsions (often referred to as Pickering emulsions). Using confocal microscopy and environmental transmission electron microscopy, we have studied the self-assembly of colloidal-sized polystyrene particles and alkanethiol-capped silver nanopaticles in Pickering emulsions. Colloidal samples of monodisperse size, when exposed to the emulsion at low concentrations, were found to form small patches with local hexagonal order; these crystalline domains were separated by other particle-free domains. Polystyrene particles with different sizes (1 micron and 4 microns) and different wettability could simultaneously segregate to the emulsion interface and form mixtures on it. In contrast to microparticles, the dodecanethiol-capped silver nanoparticles of 1-5 nm form randomly distributed multilayers at the liquid/liquid interface, with an interparticle distance varying from close contact to approximately 25 nm. Our work offers the first direct observation of nanoparticles in a liquid medium using the environmental transmission electron microscope (E-TEM).