"Ok, so anyone can use copyrighted images for their user icons as long as they make a post about it at some point?"

Technically "No", but yet many people do. In reality, no one is entitled to use a "copyrighted" image unless they receive permission to do so from the copyright holder.

"yes anybody can use the image of a well known character"

That is not true! Again, many people do this, but in reality it is illegal to do so. There are however different levels of IP rights infringement. For example, it would be considered much more serious to use a copyrighted images let's say for the cover of a book that is to be sold for a profit, than it would be to use a copyrighted images as a personal "avatar". By convention, the key is really the purpose for which the image is being used.