How to use it
Drop a picture on the page, paste one with ⌘V or Ctrl+V, or press Choose a file.
Drag over each thing you want covered. To move a box, drag it. To resize it, pull a corner.
Pick black or white. Black is the usual choice.
Press Download PNG, or Copy to paste it straight into a message.
Look at the picture under the buttons. It's the file you just made, read back from the file itself. Check that every box covers what it should, and download again if one doesn't.
Doing it well
Cover a little more than you think you need. A box that stops at the last letter can leave the tail of a g or a y showing.
Look at the edges of the picture too: a name in a browser tab, a web address, a file name, a notification in the corner. This page covers exactly what you drag over and nothing else, so read the picture once more before you send it.
Blur and mosaic are left out on purpose. Either one can sometimes be undone by someone with the right tools. A solid block can't, because there's nothing under it to find.
Redact as you capture
VizShot does this in its editor the moment you take a screenshot: Redact (E) covers what you drag over with a solid black block, and VizShot doesn't blur. Redact or pixelate? says which to use when, and you can download VizShot and try it free for 14 days.
Questions
Is my picture uploaded?
No. The picture is opened, covered and saved inside your browser, and no request carries it anywhere. To see for yourself, open your browser's network panel and do the whole job.
Can the covered part be recovered from the file?
No. Each box replaces the pixels under it with one flat colour, and the image is flattened and saved as a new file. There's no layer to delete and no copy of the original inside it. The picture under the buttons is that file, read back.
Why only solid blocks?
A blur or a mosaic can sometimes be undone by someone with the right tools, and a solid block can't. If you only want to soften a face, you can pixelate just that part and keep the solid block for anything private.
Does it remove the details hidden in the file, like where a photo was taken?
Yes. The file you download is made fresh from the picture's pixels, so details such as the camera and the location don't come along. To strip them from a picture without covering anything, use the metadata remover.
What file do I get?
A PNG, with the same width and height as the picture you dropped in.
I've just taken a screenshot. Is there a quicker way?
Yes, if you have an app. The three-step version is on the blur a screenshot page, and how to redact a screenshot has the longer answer.