[jQuery] Change all CSS background images url

[nomen]

Hi all:

    I have a website. All my images are in "/img" directory.
    Now, my client needs to put the site in a subdirectory, so, my
images are now in "/subdir/img".
    In the future, maybe we have to change the site to another
subdirectory.
    I don´t want to change all my CSS anytime the client changes the
subdirectory.
    So my question is:
        Is there a simple way to change all CSS background-image
property with the next logic:

               "Change all the existing CSS background-image
properties in this way:
                   if the url starts with "/img" change to "/subdir/
img" else do nothing"

   Thank you for your help in advance.