You can get to the Image editor when you are editing your email or on the library tab. Just click the Image Editor button
When editing a campaign
On the Library Tab
Once in the Image Editor, click Crop along the top of the window.
Once you click Crop you have a few different options on how to crop your image. If you have a layout that you need, like a square image or one that is taller than it is wide, choose on of the existing templates available across the top. You can even put in specific height and width that you need! You can also use the big white circles to adjust by eye. Take a look below to see what the window will look like. Keep in mind that anything greyed out along the edges will be cropped out of the image when you click Apply.
Click Apply to finish cropping your image, and then Save. Your last choice is to determined whether you need the original file in your account still, or if you will only need the cropped image. If you want to keep the original file click Copy in the next window; however, if you will not need the original in your account click Replace.
And that is it! Five easy steps on how to crop your image directly in your account.