Tomado de aquí.

1.-The image file is located in the following place:
C:Program FilesMicrosoft CRM ServerCRMWeb_imgsmasthead.jpg
You could copy another image with the same name within that folder.

2.-Also, you could change the .CSS definition, which is a TD, within:
C:Program FilesMicrosoft CRM ServerCRMWeb_commonstylesglobal-dynamic-styles.css.aspx

The TD name is:
Then, you could change the name of the image’s file within this TD definition. (in case you want to use a different image file).

Of course, I warn you that anything you change within CRM files could be re-applied when installing service-packs, etc., so be careful with this kind of changes…

Because of this, it is a clear unsupported stuff, even though it is a very simple customization.

I’m quite sure we’ll have it as a regular customization in future versions.

So!, It is a little hidden, but, quite easy, of course. 🙂