You are only able to upload your own contact photo. You cannot do this on behalf of another person.

STEP 1: Click the face icon next to your name under Key Contacts.

STEP 2: Click on Update Your Profile in the pop-up window.

STEP 3: Click on the camera icon next to your old photo or blank face icon. 

STEP 4: Click on Upload a New Photo. Select a photo from your computer,

Please select a professional-looking headshot of you. No pets, cartoons, or avatars.

STEP 5: Your photo will update on your Delve profile and on Key Contacts.
There will be a delay between the time you update your photo on your profile and when it shows up under Key Contacts.