Hi ,depends on what you want to study..like if you need to study histogenesis ,techniques used should be same for all slides. (magnification ,thickness of tissue, same camera settings) you take images n can compare cells/ basic unit structure like a cells / follicles per field . If you want to measure thickness of structure, can use optic gratitude while studying slide.
histological analysis is qualitative based on morphology and observers pre-existing knowledge of structure and identification of abhorrence , look up shotgun histology on youtube and check out the videos from washingtondeceit, He's awesome.
I did alot of imaging of lung tissue a while back and found this program was invaluable for stitching images together http://www.cs.bath.ac.uk/brown/autostitch/autostitch.html providing your original images are of good quality.
Personally I edit my photos in ms-paint still as all i need to do is add a few arrows and comments etc