Is a picture worth a 1,000 words?
My first reaction is that the heart symbolizes the fragility of human nature, and how like dried leaves our experiences can either pile together until we almost overflow, or they can leave us brittle and broken.
But that has nothing to do with gratitude, now does it?
The artist here, BidWiya, had in mind an old proverb: who does not thank for little, will not thank for much. According to the artist's page on DeviantArt, he says that the leaves symbolize that ‘the days will pass, the leaves will dry and I have still not given you enough to say thank you’.
Which is beautiful, really.
Have you let those closest to you know how much you appreciate them? Or are the leaves of your life crackling, drying and slipping away? Maybe it's time to turn the computer off (I know - *gasp* - put your writing away) and spend some time with the person you most appreciate. I'm off to do just that.