The website, along with Moodle, ACDATA, and other systems, uses the photo that you upload to the campus directory (directory.amherst.edu). Here's how to change that photo.
Go to the campus directory and sign in.
Click Edit my profile.
Under your current photo, click Update photo.
Click Upload new photo.
Either drag a new photo into the image box or click on the box to browse for a photo to upload. Optioinally, you can choose to use your ID photo.
Once uploaded you can see previews on your photo in the various formats used in different college systems. Drag the photo against the plus sign ("+") to change the center point, and use the slider to change the zoom. Once your satisfied with how your photo looks in all the previews, click Save Changes.