Note: @C.Monsour's comment should be the definitive answer to this question. Capone was not a fugitive. If @CMonsour will provide that as an answer, I'd urge all to upvote it and urge OP to accept the answer as authoritative.
I am not a lawyer, but I interpret "aiding and abetting" differently.
Aiding and abetting is a legal doctrine related to the guilt of someone who aids or abets in the commission of a crime (or in another's suicide). Wikipedia
In all cases of aiding and abetting, it must be shown a crime has been committed, but not necessarily who committed it. Ibid
When he was sitting for a photograph, was he committing a crime? If not, then the photographer was not "aiding and abetting". (Aside: I seriously doubt that they called Capone in to sit - Newspapers go to the story, they don't ask the story to come to them).
Did they send a photographer to take the headshot, or did they use a stock photo? Or did they purchase the photo from a stringer? If they invited him to the office, then there might have been counter-charges of entrapment, although I don't know how Illinois dealt with that at the time.
The question suggests that they should have arrested him. Still not a lawyer, but the power of citizen's arrest is very limited. I believe this power would be governed by state law, and I'm not familiar with 1930's Illinois law, but the Wikipedia summary is:
In the United States a private person may arrest another without a warrant, for a crime occurring in their presence. For which crimes this is permitted may vary state by state. Wikipedia
Once again, if the photographer didn't observe the crime, then the citizen's arrest might not be legal; I even think it is plausible that it would complicate police's attempt to arrest Capone.
Finally, one must consider
Freedom of the press. @Devsolar's answer on this issue is superior, although I wish it explicitly called out to the First Amendment's protection of Freedom of the Press, which would make it very difficult for the government to prosecute a journalist for performing a legitimate journalistic function.