To get to the edit screen will look a little different depending where the image is being used.
If you're under Site Setup or any of the Contact manager fields, you can click on the "pencil" icon on the photo.
Anywhere else you're looking to open up the image so that you can see the "Edit Image" link
Next Choose "Crop" from the top toolbar
On the right hand side you can choose aspect ratio.
for example, 1:1 is square. 2:3 or 3:2 are common vertical or horizontal crops
After Making your crops, make sure to save your changes