It's not possible to heart an image directly.
The Heart Button is a tool to scan images on websites. If you open an image in a new tab or window you won't be able to heart it, you must go back to the site and heart it from there.
We've designed it this way because We Heart It is a bookmark site. When you bookmark an image, we save and display the link where it came from, so that other people can visit the site and see more images and information on that page.
If we allowed people to heart directly from the image URL we wouldn't be able to link back to the original site, since it would be impossible to know which site the image came from.