I have seen many posts here from people trying to change the logo and not being able to. So I thought that I would give some handy notes here.
These notes are for MULTISITE Drupal installations. We will call our site example.com in these notes. Also, I use Linux so these notes are for Linux.
First, make sure that you have a logo-sized image. Use PNG if you can because it's license-free.
Next, ensure that you create a directory under /sites/example.com/ to store images in. We do this under your site's directory because, of course, you do NOT want to contaminate other sites!
I called mine "images".
Ensure that you chown the directory so that your web server software (apache) either owns it or is in the group for access then chmod 770 the directory. I keep all my directories owned by my Drupal master account with apache as the group. So as root I "chmod . images".
Now copy your logo into this directory. Again, ensure that you chown and chmod the file so that apache can read it!
Now in your admin acct for your Drupal site (example.com), go to administer/themes/blumarine
Check that you have bluemarine enabled and your default theme (if you want it as your default theme of course).
Now click on "configure".
Under "ologo image settings", unset "use default.
Now this is the important part. This theme needs the full *relative* path of the logo file put into "Path to custom logo". The path is relative to the Drupal root directory on your server AND must not start with a leading slash. So in my case, this is: