“Your attachment is over 2 MB.” Drop the photo in, choose the quality, save. It goes nowhere, not even the photo of your ID card.
What it does not do: Lossy compression is FINAL: what is taken out does not come back, and running the same photo through twice degrades it twice. Keep the original. Re-encoding an already compressed image does not always make it lighter: the screen will tell you, rather than handing you a bigger file while claiming the opposite. And there is no AI upscaling here: a model of several dozen megabytes would have to be downloaded from a third-party server, which undoes this site's whole argument.
Nothing is uploaded. Cut your network connection once this page has loaded: the tool keeps working. How to check it yourself.