How do I blur an object with an irregular shape?

Use the brush to paint over it, or the lasso to trace round it and let go — the outline closes itself. Either becomes a region you can move, feather or delete. The rectangle and ellipse are there for things with simple shapes.

Can I blur everything except one part?

Yes. Draw the part you want to keep, then switch on Invert for that region: the blur applies to everything outside the shape and the inside stays sharp.

What does feather do?

It softens the boundary of the region outward, over the number of pixels you set, so the blurred area fades into the picture instead of ending at a hard line. It only ever grows the region; it never thins it.

Is a blurred part safe to post if it shows a number or a name?

No. A blur over text in a known typeface can sometimes be read back, especially short strings like a plate or an account number. For anything that must not be read, switch to Privacy and use the solid fill, or open the photo redactor.

Blur Part of an Image — Brush, Lasso or Box

Blur exactly the part you mean

This mode opens on the brush, because most of the things people want to soften are not rectangles: a cluttered shelf, a reflection, a sign at an angle. Paint over it and it goes soft. The lasso traces an outline and closes it when you let go; the rectangle and ellipse drag a shape. Every stroke or shape becomes a region in the list beside the picture, with its own blur, strength, feather and invert.

These regions are effects. They are listed as effects and never given a verdict, because a blur spreads what was there rather than removing it.

Working with regions

Draw several regions; each keeps its own settings, and they are applied in the order you made them.
Select a region to change it. Arrow keys move it, Alt and the arrow keys resize it, Delete removes it.
Feather grows the edge outward so the blur fades in; Invert blurs everything except the shape.
Brush size is in pixels; the [ and ] keys change it while you paint.

When another mode is better

If the part you are blurring is sensitive — a face, a number, a name — use the photo redactor and its solid fill instead. To soften everything behind a person, use blur the background; for square blocks, use pixelate an image.

Limits, stated plainly

Nothing is found automatically in this mode; you draw every region.
A blur is reversible in principle on short printed text. Use the solid fill for anything that must not be read.
An animated GIF or WebP is saved as its first frame.
JPG, PNG, WebP, GIF, BMP and ICO open here. HEIC and TIFF open only where the browser itself reads them.

Questions about blurring part of an image

How do I blur an object with an irregular shape?: Use the brush to paint over it, or the lasso to trace round it and let go — the outline closes itself. Either becomes a region you can move, feather or delete. The rectangle and ellipse are there for things with simple shapes.
Can I blur everything except one part?: Yes. Draw the part you want to keep, then switch on Invert for that region: the blur applies to everything outside the shape and the inside stays sharp.
What does feather do?: It softens the boundary of the region outward, over the number of pixels you set, so the blurred area fades into the picture instead of ending at a hard line. It only ever grows the region; it never thins it.
Is a blurred part safe to post if it shows a number or a name?: No. A blur over text in a known typeface can sometimes be read back, especially short strings like a plate or an account number. For anything that must not be read, switch to Privacy and use the solid fill, or open the photo redactor.