Felix, the legislation is very clear. It is not enough for someone to be upset. The behaviour has to be directed at a person, it has to be behaviour which a reasonable person would consider to be harassment, and it has to happen on at least two occasions.
None of those criteria are met in this situation.
I repeat: it is not enough that someone is upset. You cannot commit harassment accidentally because an unspecified person happens to be upset by something they happen to see you doing in your daily life.
Now perhaps the person who reported the sticker incident to the police is genuinely upset. If that is the case, they need to seek professional help to enable them to become more resilient so that they can function in society. The police taking their complaint seriously is more likely to hinder that process than to help it. The police should have explained to this person that they need to be able to exist in society, along with people who do not necessarily share their views or their taste in stickers, and that they can't expect the police to go round having words with everyone who doesn't live in their rainbow coloured bubble.
More likely, this person is not genuinely upset, but is simply a malicious complainer who wants to cause trouble for people who do not live in their rainbow coloured bubble by reporting completely reasonable and legal behaviour to the police, because they have learned that the police will not tell them to grow up and stop wasting everyone's time, but that they will collude with them to make life difficult for people who do not share their views by acting as though photographing stickers is a police matter when in fact it clearly is not.
You are really not helping to counteract the impression many people now have that the police are an absolute disgrace, and that they are neither competent nor impartial.