Who established that the creation of propaganda is exclusively limited to artists working in government agencies and that private artists cannot make propaganda?
Propaganda is not a word used to describe independents making a political statement, then everything political would be propaganda.
In my opinion, propaganda and political agitation are very similar - if a report or statement aims to sway someone into taking some political position towards one thing or another, than it's propaganda. If an American news channel broadcasts a report stating that totalitarian regime in Russia is oppressing the free people and proclaiming that America must take action against Russia, it is propaganda. If a Russian news channel broadcasts a reports stating that America is interfering in Russian affairs and proclaims that Russia must defend itself against America, than it's also propaganda. Hell, even this post is propaganda, because I try to propagate my political views here and convince you that they are right. If your horse is hungry, bring him here, because there'll probably be a lot of straw here.
Unbiased reporting and propaganda are
not mutually exclusive. Take for example BBC's coverage of the war in Syria: articles tell us about the official position of the Syrian government, so we may say that it is unbiased. But, the propaganda content is hidden in the articles' wording, for instance many claims stating that the Syrian Army commits war crimes are attributed to "activists". Many articles don't tell us about the political allegiance of activists, so one may assume that these activists (or at least some of them) are independent and thus unbiased. If the activist is unbiased, then his claim is very likely true (he's throwing a serious accusation, why would he lie about that?), and the official position of the Syrian government stating that Syrian Army doesn't commit war crimes and that rebels commit war crimes is very likely false. But, the information attributed to activists usually comes from opposition activists and organizations affiliated with the opposition, like the Syrian Observatory for Human Rights. Their information is biased, like all reports written by one of the belligerents about their enemy, but thanks to their political allegiance being left out, it was presented as unbiased.
Western media generally don't mention the allegiance of political forces and organizations they like - pro-Western activists and followers of pro-Western political organizations are presented as members of civil society or simply activists, while pro-government activists and followers are described with terms like "government supporters" or "supporters of [insert name here]'s regime". Sometimes, the political status of anti-government followers isn't provided at all, and protest against the government organized and carried out by members of a political organization is presented as a protest by ordinary people. It is used to imply that the pro-Western opposition has more popular support than the government, and that the opposition members are more unbiased and honest than the government supporters, which are basically apologetics.
Another hidden propaganda method is generalization. If A happens to a few people representing (or claiming to represent) a group of people, then A happens to every single member of that group. If a gay rights activist is beaten by hooligans for whatever reason, then the absolute majority of gays in Russia experience the same. As a result, we have all reports in the Western media about homosexuals being cruelly persecuted in Russia, which are based on incidents that happened to a group of people taken and presented as usual occurrence to all Russian homosexuals. BBC reports about civilians being killed in Syria by collateral damage by taking interviews from civilians that ultimately retell the claims provided by the opposition. It is implied that the majority of other civilians on the ground experience the same. The people interviewed might not even be actual civilians, maybe they were paid to pose as ones, but it doesn't matter. The information is presented to reinforce the opposition's claims, and not to tell the unbiased story about the impact of the war on civilians.
Russian columnist Viktor Marahovsky from the staunchly anti-Western and anti-liberal Russian website
"Odnako" (one of the biggest and probably one of the most moderate Russian anti-Western news portals) rather unsubtly named such wording practices "Demospeak"
(his Russian article can be found here; like all articles on the website it is propaganda that is not disguised as unbiased)