This site includes a virtual microscope tutorial, infomation on making hisology slides and photomicrographs accompanied by some good test questions relating to the image. This is a searchable database with zillions of images. It also includes pathology images. Very nice, high quality images. There are histology specimens from many species.
People contribute their images to the database.
If you know your stuff, don't skip this site. This is an entire histology course. There are images, lecture notes, lab notes a microscope tuorial, quizzes and even more links. Lecture material is alongside images. The images are labeled so you know what you are looking at.