When you try to heart images using the bookmarklet some images may not get the pink border, that means they can't be hearted.
To be hearted an image must meet all the requirements below:
1. a width that is greater than or equal to 240 pixels
2. a height that is greater than or equal to 200 pixels
3. a valid image extensions (jpg, png, gif)
On the image above you can see one of the images on the site didn't get the pink border because even though the width is bigger than 240px the height is not.
If no valid images are found on the page you'll see an alert window saying that no images were found.