You cannot change your Gmail account profile picture separately from your Google account profile picture. The reason for this is that there is no such thing as a Gmail account, only a Google account. Gmail is merely a service you can use under your Google account. That is also why you cannot open a 'Gmail account' without having access to other Google services as well. Therefore, when you change preferences by clicking on the link to your profile on the Gmail interface, it directs you to the Google account page. I am not aware of any method that enables you to only change your profile picture, or indeed anything on your profile at all, for Gmail and Gmail only.
I know it was doable because my profile pics on my two Gmail accounts are different from each other AND different from my overall Google profile pic.
As for the above part, I'm not sure I understand. Perhaps (and correct me if I'm wrong) you were talking about having 2 Gmail addresses under the same Google account? That is, in fact, possible. However, since they are still both tied to the same Google account, it is impossible to set two separate profile pictures for both.