Is it appropriate to Post the pictures of the same subject with subtle differences?

One thought: if you are submitting two images that are different, but might seem similar, be sure that the image description gives the full difference in each case. Moderators may use the description. (Equally they might not, and may not have a common language). End users who search will also be clued in by the title. They like "spam" even less.  

If they truly do display the subject differently and are each unique, you can certainly upload each one; but I wouldn't do them all at once. Upload one, wait for it to get accepted, then another later. 

It seems to me that if you have acceptable images that display singnificantly different aspects of the same subject, then submit them.  But if the differences are subtle (as you describe them), then you run the danger of having your images be considered "spam" and that is an abuse of the system that can get you banned.   I think it wiser to be selective about what (and how many) images you submit..