When you are uploading images from your phone or camera they are usually too big.
WordPress automatically creates small, medium, and large image sizes for your original upload. But sometimes you may need to crop the image in a different size.
WordPress is no Photoshop, but it comes with some basic image editing features like scale, crop, flip and rotate images.
Go to Media » Add New and upload your image. After uploading the image, you will see an Edit link next to the uploaded image.
Clicking on it will open the Edit Media screen where you will see an Edit Image button right below the image. This will open the WordPress image editor.
On the image editor screen, you will see buttons to perform some basic image editing tasks such as cropping, rotating, resizing, etc.