While I am not a lawyer and this is not legal advice, I don't think we need one to know that using other people's content is illegal in most countries. Doing so is a blatant copyright violation.
To post anything you must either own it or have permission to do so. In some cases images on the web are posted along with a license. The licence tells viewers what they are allowed to do with the content.
There exists various Creative Common licenses which let people share content under such licence with some restrictions. See the link for details. Most content which is freely shared requires that you do the same. So using free content for a commercial purpose is rarely possible.
You can never assume the license of any content. The vast majority of content belongs to its author. If content is published under a certain licence, it will be explicitly stated.
There is also some content which falls into the public domain. Depending on where you live, this may be content whose author died more than 75 years ago for example but this refers to the original content. For example, if a music composition is public domain, the recording of someone playing that music is not public domain.
For completeness, there is something called fair use which says that you can use part of a work for some purposes. A typical example is extracting some text to quote a book.