How do you change the size of a picture?

i have a picture that i need to be 18.2cm high and 27.2cm wide how can i do this or what program do i need to do this if possible could it be a free programHow do you change the size of a picture?
try

http://www.resizr.com/How do you change the size of a picture?
Photoshop Elements about $100.00 but well worth it!