Dataset of microscopic images before and after staining?
Virtual fly brain (http://www.virtualflybrain.org/site/stacks/index.htm) is an excellent database of fly (Drosophila) brain sections, which can be electronically processed/annotated/highlighted on the fly to show different areas of the fly brain as an overlay on top of the original images.

Alternatively, you can use BrainTrap (http://fruitfly.inf.ed.ac.uk/braintrap/) for the expression of proteins in the fly (Drosophila) brain using brain sections. Another database which has silver stains of Drosophila brain sections is the flybrain (http://flybrain.neurobio.arizona.edu/Flybrain/html/atlas/) database. This database also contains 3D models of different fly brain structures and much more!

I'm sure there are many many similar databases, but these are the ones I knew of, which are widely used in the Drosophila community.