Tested, dated, first-person. Four sections — pick the one closest to your question.
If you arrived because something rejected your file, go to Tutorials — the “compress to an exact KB” guide solves the most common reason people land here. If you are preparing images for a website and care about page speed, start with Formats & performance. If you are about to share something private or use a tool you have not vetted, the Privacy & security section is the one that might change what you do. The Behind the scenes articles explain how the site itself works, for readers who like to know who they are dealing with.
Every guide runs the same way: the work happens in your browser, the files never leave it, and the author is named at the top.