"If it doesn't have a copyright notice, it's not copyrighted."
You should always assume that other people's works are copyrighted and may not be copied unless you know otherwise. Copyright will apply whether there is a copyright notice or not. Whether it is literary, musical or artistic there are penalties for copyright infringement.
"If I don't charge for it, it's not a violation."
False. It's still a violation if you give it away and there can still be damages awarded if you hurt the commercial value of the property. Except in specific circumstances permitted under fair dealing/fair use rules, any copying or publication without the consent of the copyright owner is an infringement, and you could face legal action.
"Crediting the content creator protects you from liability."
Similarly, how often have you seen this disclaimer? “I do not own this content. All rights belong to the original creators respectively. No copyright intended.”
This is a great example of how many people misunderstand copyright law.
"If it's posted on-line, it's in the public domain."
False. Nothing modern and creative is in the public domain unless the owner explicitly puts it in the public domain.
"It’s hard to prove copyright infringement."
This is not the case, copyright law is principally civil not criminal law. Civil law requires a lower burden of proof, actually making it easier to prove infringement.
"Content on social media is in the public domain."
Unless you’ve been given permission by the original creator, it is very likely you are infringing on someone else’s copyright.
"“I've removed the copyrighted material” protects you from any consequences."
Simply removing their content cannot absolve you of liability for infringing on someone’s copyright.
What can you do to protect yourself?
Use stock content from sites like iPHOTOS.com, iCLIPART.com, AnimationFactory.com and Clipart.com or get explicit permission from the creator and don’t assume images online are fair use.