Paint on this site has no Crop command in its Image menu, but cropping works through the Select tool and the picture size. There are three ways to do it: drag a box with the Select tool while you hold Ctrl, make the canvas smaller so only its top-left part stays, or copy one area and paste it into a fresh picture.
Crop to any rectangle with the Select tool and Ctrl
This method works for any rectangle on the canvas. The part that matters is the Ctrl key: keep it pressed for the whole drag, right up to the moment the mouse button comes up.
- Click Select in the tool box, which sits at the window's left edge. This is the rectangular one, not Free-Form Select.
- Press Ctrl and keep holding it.
- Drag a box around the region you plan to keep.
- Release the mouse button first, and only after that lift your finger from Ctrl.
Check the cropped picture and undo it if needed
When the button comes up, the canvas shrinks to exactly the box you drew, and everything that was outside the box is removed. If you framed the wrong area, press Ctrl+Z. Paint treats the whole crop as one undo step, so a single press puts the full picture back and you can drag again. If the canvas kept its size, Ctrl was no longer down when you released the mouse, and the area is only selected. Press Enter or Escape to commit it, then repeat the drag, holding Ctrl until the mouse button comes up. Free-Form Select does not crop.
Keep the top-left part by making the canvas smaller
When the region worth keeping starts in the upper-left corner, you can crop by typing a size instead of drawing a box. Paint pins the existing content to that corner, so a smaller canvas trims away the right side, the bottom, or both at once.
- Open Image > Attributes..., or press Ctrl+E.
- Enter a smaller number in Width, in Height, or in both boxes.
- Press Okay to apply the new size.
Trim the canvas by dragging its edge
You can get the same result by eye. Drag the resize handle on the right edge, the bottom edge or the bottom-right corner of the canvas toward the middle. As with the Attributes... dialog, the top-left region is what remains, and whatever lies past the new edge is removed. The picture size can't go below 1 by 1 pixel.
Copy one area into a new picture
This method builds the cropped version as a separate picture, and it can also save the chosen area straight to a file. Here the Ctrl key is not used while you draw.
- Click Select and drag over the region to copy, without holding Ctrl.
- Press Ctrl+C to copy it, or choose Edit > Copy To... to save just that area as an image named selection.png.
- Start a blank picture with File > New.
- Press Ctrl+V to paste the copied area, or choose Edit > Paste From... and pick the saved file.
Use the Crop button when a pasted image is too big
Pasting an image that is larger than the current picture opens a question: should the canvas grow to fit it, or should the image be cropped? Choosing Enlarge grows the canvas to make room for all of the pasted image. The Crop button in that dialog leaves the canvas at its current size instead, so any part of the pasted image that falls past the canvas edges is cut off.
Finish a selection so the picture looks whole again
The selection tools lift whatever is inside the box the moment you draw it. Until you commit the selection, the spot it came from on the canvas looks cleared. Press Enter or Escape to commit the selection and put those pixels back into the picture.