Cropping is often used to fit an image into a limited space or to get rid of unwanted parts of a photo. Typically, cropping is done with a rectangle but it can also be done for other shapes. The process is similar but has a couple of slight differences. Let's see how you can crop a regular rectangular image into a circle.
Promote the background
If your image is a background, it won't let you have a transparent area as it will automatically fill anything you delete, with the background color of the Materials palette. So if you have a background layer, right-click on it, and choose Promote Background layer. That is when you will see the layer's name change to Raster.
Make a selection
Activate the Selection tool (it shows as a little rectangle and is typically the third icon in the tools. If you don't see it, it might be visible by clicking the little triangle on the side.
Once you have it active, go to the settings and under Selection type, choose Circle (or Ellipse)
Draw your circle
Although it might be intuitive to start your selection on a corner, the Circle selection is created from the center so you need to guess where the center of your subject is. If you are not exactly sure, don't worry as you can move it later. Just make sure you don't make a selection that will touch the edge because it will then show a straight line instead of a neat curve.
Adjust your circle
If you need to adjust the placement of your circle, you can right-click on the selection and move it where you want so you don't have to redraw it if it is off.
If you find that the circle is too large or too small, you can edit this with the Edit Selection command. You can find it either under Selection > Edit Selection, or by clicking the icon in the Layers palette. This will turn your selection to a pink area that can be manipulated just like any object, using the Pick tool. Once you are happy with the size and placement of the circle/ellipse, click on the Edit Selection again to make the familiar "marching ants" reappear.
Under Image, simply selection Crop to Selection, and you are done.
Now you can save your new image as a png format, making sure that you have set it to keep the transparency.
And you can use the exact same steps to crop in another shape, whether it is a triangle, a hexagon, and others.