On your heart page, you can see all the images you've hearted. 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.
If you would like an image you have uploaded permanently removed from We Heart It, and can prove that you are the owner of the image; you will need to claim copyright. To do so, please fill out the following copyright form.