In the example I found, it usually uses one images for a particle system. Is it possible to use multiple images in a system?
Particle system can only use one material type. The particle system can, however, alter the tint and transparency (obviously) of specific particles as well as have an arbitrary number of emitters. To get more complex effects, however, you will need to combine different systems.
I do not have experience with Ogre3D specifically but maybe you can make a texture with multiple images inside and replace the UV's matching the image you want?
You can write wrapper around your Ogre particle system which represents internally two particles systems with the same parameters but with different images, and call it instead direct call of Ogre3d Api.