Because they're cigarettes.I hate people who do that. "new pic =) it in grayskul cuz im so kool"
I think it's just to create a fitting mood for their character.
I've heard that they sometimes make it grayscale to hide imperfections in color.
I didn't realize "old" was a "character mood".
if the photo is of their face, converting color images to grayscale can easily cover up blemishes, zits, et cetera, or make it more easy to cover them up with a simple editing program.
Congrats Sirrus, you are now cool and artsy.