Or you could *gasp* write "from xkcd.com" onto the printout yourself. I know, this is tricky, but I'm here to help, with three easy-to-follow guides.
The 'Handmade with Love' Style:
1: Print the comic
2: Get a pen
2a: This may be in a drawer
3: Write "xkcd.com" somewhere on the printed comic
3a: Don't write over any of the pictures
The 'Look, Ma, I Can Use Paint' Style:
1: Download the comic image
2: Download the site logo
2a: This is the thing in the lop left corner of the forum pages
3: Open the comic in an image editing program
3a: It's the thing in Start > Accessories > Paint
4: Open the logo in the same program
5: Resize the logo, and put it in a corner
The 'Hey, Now I'm Actually Useful' Style
1: Learn Python
2: Download the PIL
3: Write a script that can automatically download any (or all) of the comics and then add the logo to a corner of them
4: Post it in the 'Coding' subforum
5: Print one of the comics you've magically created with your newfound haxxor skillz.
Now you can become a productive member of society, along with carpenters and mechanical engineers and those cool bacteria that eat spilled oil.
As long as I am alive and well I will continue to feel strongly about prose style, to love the surface of the earth, and to take pleasure in scraps of useless information.
~ George Orwell