The background artists usually use arcylic or
watercolor, depending on the scenes.  Just go to
your local art store, each brand of paper usually
lists which type of art medium is suitable for
painting on.  Or you can ask a salesman there to 
help you.

Actually in your case, the most easiest way for 
you is do it on computer if you have the 
softwares and hardwares.  You'll need a scanner, 
color inkjet printer and a photo editing tool 
like Photoshop or Paint Shop Pro.  Basically, you 
find a real photo, scan it, then opens 
Photoshop.  It has many filters that you can 
modify the photo, like in watercolor or pastel.  
Then you print it out by using the color 
printer.  When you're familiar enough with 
Photoshop or other art programs, you can paint it 
yourself on computer, then print it out.