If you are using any built-in site master page as your default master, Go to Site Action—>Title,Description, and Icon–>Change the Value of Logo URL and Description will not work.
First, off I wouldn’t edit an existing master page definition. Copy it (or file; save as) instead and call it something that makes sense to you remembering to keep the .master portion. I’m assuming you’re doing this after pulling up your site in Designer and going to _catalog/master pages. That’s where they need to live.
Once you’re editing it click on CSS Properties tab in the lower left properties panel. This will tell you which css file is being used to define fonts styles colors & logo files etc on that master page. For BlueBand its blue.css located in the \Style Library\en-us\core styles directory. Copy that too and change its name.
When your new master page is open you’ll need to go into code view to find any references to the old css file, normally just one up at the top of the document but I always do a find/replace when doing this sort of thing so I don’t miss anything.
I put my image files in the proper location before I make my edits. That location is \site library\images.
Once that is done you can doubleclick on the css (zz1_black.css) file to open it. There are two areas that need changing in the CSS file. They are:
You’ll just change the bl_band.jpg and the bl_logo.jpg to your image files. Make sure they are sized the same way as the original or your page layout will get messed up.
Save your changes, close your files, check them in, publish them via the master page gallery (if you have approvals on) and then change the master page for the site collection under site settings. Voila – new look!