1. Select the image and click on the Editor icon
2. Change the dimensions of the image by dragging left or right on the scrub bar (highlighted in Red)  
3. Drag the orange box outline over the image and select 'Apply'
4. This will return you to the main editor screen where you will be advised of the new file name. Select 'Apply'
5. Save the change in the Editor. If the image doesn't appear (e.g. you see an Image Placeholder asset instead of the edited image) refresh your web browser