I drew a complete blank with this theme at first, and just happened to notice a couple of potential shots while I wasn't really looking. This one I had taken a few weeks ago, but luckily the graffiti was still there so I went back and took another shot especially for this project. I liked the positive message rather than the usual swear words and 'woz ere'!
I brightened the image and then added the Orton and Holga effects to enhance the contrast and darken the edges. I think the image fits the theme, not only because it is black and white, but because the 'light' message of smile is a contrast to the 'dark' setting. Not explained it very well, but that's how I see it!
Some reject shots:
I was cooking/burning some toast the other day as the sun was shining through my kitchen door, and the sun lit up the smoke (don't worry, there wasn't much smoke!). I used a longer exposure and then increased the contrast on Picnik.
And here is the black and white version.