While @jqning is absolutely correct in stating that truth is always an "absolute defense" to a claim of defamation, keep in mind that truth can be a subjective thing. What is one person's version of the truth, may not be another's, even with regard to the same exact experience. Also, while "statements of opinion are not defamation" is typically regarded as true, it has very broad exceptions and is not something that can be relied on in isolation.
Defamation is generally defined as a false, published statement that is injurious to the plaintiff's reputation. An online posting, even on an obscure website, will likely be seen by a few people, thus satisfying the publication requirement.
A plaintiff cannot succeed in his or her online defamation claim if the defendant's defamatory statement is true. So, for example, if a customer posts a review of your restaurant on Trip Advisor claiming that there were roaches crawling around, you may sue them for defamation. You would then have to prove that there was no roach infestation, and thus, the defendant's statement was false. However, what if there was only one? What if he has a witness who saw it? His truth may be different from yours, and it is up to the trier of fact to decide. Also, getting sued, whether or not you prevail, is at minimum a pain and can be a very expensive ordeal.
Opinions are exempt? OK: Following that line of reasoning, restaurant owner shows he's had monthly inspections and prophylactic measures to ensure against pests and the exterminator testifies. The defendant, fearing he's in trouble now, claims that his assertion of roach infestation was just his opinion based on his experience. Opinions are privileged under the law of defamation, right? Not always!
Importantly, an opinion may be viewed, generally, as a statement of fact (employing the "reasonable person" standard) if it is something that is either provable or disprovable. What this means is that if the reasonable person would construe your statement to be factual, and not mere opinion, it will be deemed as such and if untrue then you're liable for defamation. The courts may interpret, "I think that [restaurant] has a roach infestation problem," as a statement of fact.
This has occurred in numerous cases where people think they can say what they want as long as they couch it as an opinion, with words like "I think..." or "In my opinion...". But when someone says something that factual in opinion form, that is not protected. So, if Jane says, "In my opinion Joe Schmoe is a pedophile..." without absolute proof that Joe is, in fact, a pedophile, then this is libelous (defamation if published or spoken to another). This is because the statement in and of itself is one of "verifiable fact couched in opinion" and it is so damaging to Joe's reputation that if it's not true it is libel per se (defamatory if published – meaning shared).
A statement of verifiable fact is a statement that conveys a provably false factual assertion, such as someone has committed murder or has cheated on his spouse.
While the law varies some, and sometimes substantially, from state to state, here are some often used examples arising from California courts.
Libelous (when false):
- Charging someone with being a communist (in 1959)
- Calling an attorney a "crook"
- Describing a woman as a call girl
- Accusing a minister of unethical conduct
- Accusing a father of violating the confidence of son
- Calling a political foe a "thief" and "liar" in chance encounter (because hyperbole in context)
- Calling a TV show participant a "local loser," "chicken butt" and "big skank"
- Calling someone a "bitch" or a "son of a bitch"
- Changing product code name from "Carl Sagan" to "Butt Head Astronomer"
Since libel is considered in context, do not take these examples to be a hard and fast rule about particular phrases. Generally, the non-libelous examples are hyperbole or opinion, while the libelous statements are stating a defamatory fact.
Modified photos that can be shown to scandalize persons or businesses are clearly defamation, and are quite popular on social media. So, for example, if you threw the flyers (I assume you didn't but as an example) all over, and then photographed and published your opinion about the business littering neighborhoods, this would be libelous. The less obvious and absurd the modification, the more likely it is that a court will find it defamatory. So, a picture of a woman with a man's naked torso photoshopped on will not be defamatory, a version photoshopped showing what is to be purported to be her naked body, is.
In your case, you face two issues that you should ask yourself: Is your opinion really verifiable (or non-verifiable) facts couched in words that try to make it opinion, or is it truly just your opinion. If fact, is it absolutely true? If the answer is yes, it's fact and yes, it's absolutely true, you're OK. Keep in mind though what I mentioned about truths differing: What if the business didn't know they were put there, or, what if they were placed on cars in a public place and blew in the wind? That could be a problem.
While you are most likely fine, you may want to just say, X business's fliers are all over the place, littering the neighborhood and (assuming you called and asked them to pick them up, or wrote them) they refuse to pick up the litter. It sounds like the statements you made are fine, because you don't say that the business littered, or that they put them there; you say they are "plastered" all over, but you don't accuse them openly. That isn't to say it wouldn't be found to suggest fact that they would have to show isn't true (or that they didn't get permission from the property owner).
My point is only that, in general, be careful. If he felt that you misrepresented what he did by way of distributing fliers, or if he thought you doctored the photo or set it up, he could sue you if he felt it damaged his business's reputation.