Why can't I add a logo to my site?

I'm trying to add my logo to my site and it's not working. If I add it as a gif file it gives me the error message: "The change you wanted was rejected. Maybe you changed something you didn't have access to." If I add it as a tiff file, it says it adds it but then it doesn't show up on my site. I also tried changing it to a jpg file. This looked promising because the picture showed up on the accounts page and I didn't get an error. But then the logo still didn't show up on my webiste.

Any help would be greatly appreciated. I'm out of ideas.

Amy