Step 1: Choose a picture
Find a picture that could be improved by cropping and double-click to open it. It should automatically open in the Preview application.
Step 2: Select an area
Click the Select box in the toolbar. The default tool is the Rectangular Selection tool and if you click and drag round an area, you will see a marquee appear.
Step 3: Resize the box
Drag the handles at the corners and edges of the marquee to change its size. When you’re done, select Tools>Crop or press Command+K.
Step 4: Crop the picture
The area inside the marquee will be kept while the rest is discarded. Save the image under a new filename so as not to overwrite the original.
Step 5: Use a different tool
Take the original image, choose Elliptical Selection under the Select tool and select an area. You can hold the Shift key to constrain it to a perfect circle.
Step 6: Reposition crop area
Instead of dragging the resize handles at the corners and edges, you can drag from the centre of the selected area to reposition it.
Step 7: Crop the image
With your circular area selected, choose Tools>Crop as before and you will be left with a circular image rather than a square one.
Step 8: Use the Lasso
The Lasso tool can be used to draw custom selection areas. Click in the image and draw freehand around a shape, just like if you were cutting with scissors.
Step 9: Cut it out
Choose Edit>Invert Selection, then press backspace to erase everything except the area selected. This is a good way to cut things out in a freehand manner.