The most powerful and useful feature Cropping Image. Actually everyone will know how to use this application. It's quite nice and easy to understand. In case you do not know, I will tell you.
Sometimes you need a particular part of your image, not the whole of your image. Here you need to crop your image.
Let me tell you the most useful feature in here, you can adjust your cropping area (usually rectangle) by moving its edge (similar to photoshop free transform selection). Nice right?
Now you've got the area. Pressing Command-K to crop image inside that area. And if you want to overwrite that image to the original one, just use Command-5.
However, you can also save your new image with another file name, Command-Shift-5 will do this for you. You can change lots of things here.
Hooray, now you can easily crop your image.
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