New writers, here’s an article you should read. It’s from Nathan Bransford, an industry pro.
Here’s a warning that free services are not really free. You are paying for publicity, publication, storage, and more when you contribute anything online. Considering that these perks have real value, it is still worthwhile to post content of all sorts, but you should be aware that any of these sites can change their terms of service at any time. Be careful when posting recognizable photos of anyone, anywhere and think twice before using “free” cloud storage or exchange sites for potentially valuable original text. They might be okay, but I prefer external drives.
Here’s a discussion about picture books, YouTube, and copyright. Read the comments and consider your position.