Clicking the Edit Image button launches the image editor sub-panel, giving you access to several useful image editing controls. The edits you make to your image are non-destructive and you can restore the image to it's original (uploaded) specifications at any time. Here are the various controls available to you in this Edit Image mode:

To crop the image, first click on the image and drag to make your selection. Once a selection has been made, then click the crop icon above the image to finalize the crop. You can use the Aspect ratio and Selection option to make further adjustments
 before clicking the crop icon.
Rotate counter-clockwise
rotate the image 90 degrees counter-clockwise.
Rotate clockwise
Rotate the image 90 degrees clockwise.
Flip vertically
Flip the image upside-down.
Flip horizontally
Flip the image left-to-right.
Remove or restore your last edit. You can click as many times as you want in succession to "step" through the edit history.
Scale Image
Proportionally scale the original image. For best results, scaling should be done before you crop, flip, or rotate. Images can only be scaled down, not up. The original dimensions are displayed as a guideline above the box for entering new dimensions. Used in combination with the Crop icon, this tool allows you to make the following precise adjustments:
  • Crop Aspect Ratio: You can specify the crop selection aspect ratio then hold down the Shift key while dragging to lock it. The values can be 1:1 (square), 4:3, 16:9, etc. If there is a selection, specifying aspect ratio will set it immediately.
  • Crop Selection: Once started, the selection can be adjusted by entering new values (in pixels). Note that these values are scaled to approximately match the original image dimensions. The minimum selection size equals the thumbnail size as set in the Settings Media Screen.
Restore Original Image 
If changes have been made to an image, the Restore Original Image button allows you to discard any changes and restore the original image. Previously edited copies of the image will not be deleted. Press the Restore image to complete the action.
Use this to save the changes made to an image and return to the Edit Media screen for this image.
Use this to cancel any operation that has been performed on the image in the current session