This is an easy one!, but in any case, it is quite useful if you do not know how to find it. 🙂
So, the default logo image is the following:
The image file is located in the following place:
C:\Program Files\Microsoft CRM Server\CRMWeb\_imgs\masthead.jpg
You could copy another image with the same name within that folder.
Also, you could change the .CSS definition, which is a TD, within:
C:\Program Files\Microsoft CRM Server\CRMWeb\_common\styles\global-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. 🙂