First, click on Photo. Select your image. I will be using a Abraham Lincoln Dollar.
Then you will want to click on the Reform icon as shown.
Then you will want to click on the Crop icon as shown. You can also click the Rotate icon if your image is not properly oriented.
Now drag the corners and sides in to show just the coin. Click the Checkmark icon when you're done.
Now click the Save button.
It will default to a jpg image which is what you want. Click on the Medium button. High is overkill and not needed. Images must be under 300KB, see what it says under the quality slider after clicking on medium. If it's over 300KB just adjust the height or width of the image down. You can change the width and height here by using the boxes provided. Generally 800 on the longest side is plenty. You can give your image a meaningful name here also so it can be searched later.
You'll see my image dropped from 341KB to 174 KB after selecting medium. Once you hit Download your new edited image will be in your download location