The one you keep adding has poor lighting, so it's not suited to be his profile picture.
The next time you want to change something as important as a profile picture, please leave a message in the talk page and wait for a reply. If you don't get a reply and/or the replies are against the change, don't change it.
I forgot to add "for a profile". We search for specific things in a profile picture - the character must be the main focus of the image. This is the key factor in deciding that the only half-way decent picture for Doc is the one currently in use.
Look, if you feel I'm being too rigid, feel free to make a post on the talk page of Zak's article, or even the forum (in the appropriate board, of course). If the majority of members (in sufficient number) agree with you, feel free to change the picture.
First we need to discuss it on the articles' talk pages, as per the rules, so that users can voice their opinions and we don't end up with an edit war. Same reason I reverted your edits to some of the Forever Knights' articles.