Many thanks for your questions.
The answers are the following:
a. What is the subject in the images? These are photos of people who belong to different non-state armed groups (guerrillas or militias)
b. Are the images very similar even though they are labeled differently? I frankly do not know. This is the very sense of trying to find a classifier that is able to tell them apart. My hypothesis is that they are different enough to be separated by a good classifier.
c. How large is the training set? I used 400 pics for each group --that means 800 in total. Maybe this is too small?
d. Is there a large class imbalance in the training set? No imbalance
I am attaching two examples (one for each group) of the type of photos I am dealing with (by the way, retrieved with WebImageSearch).
I will give a hard look at the Classification function documentation, though I have already know it (I think) quite well --thanks again