Step 1: iCloud login
Head over to www.icloud.com and log in using your iCloud credentials. Once you’ve logged in, click on your name in the top-right corner of the screen to bring up your iCloud options in an iOS-themed window.
Step 2: Select a picture
Click on the Edit button on your photo and then click Choose to select a new image. When you’ve uploaded it to iCloud, you can resize and move the image to get it in the right place; then click Done to save.