If you want to heart an image on the site, just move your mouse over the image and click on the heart icon.
You can find the heart icon next to the image title on the image page.
When you click the heart icon, it will fill up, indicating that you've hearted that image.
. Un-Hearting images
You can see all the images you've hearted on your heart page. If you move your mouse over the image you'll see the heart icon. Click on it, and it will get outlined, indicating that you've un-hearted that image