Hi Carmen,
If you only want to change the logo in the masthead. You can do it by uploading the company logo in the Custom theme and assign this to the Master Rule collection.
Steps involved.
1) Copy the default theme and create Custom theme.
2) In the Custom Theme browse Company logo for the property 'URL to Logo'.
3) Copy Standard Ajax Framework Page and paste it in your Custom folder.
4) Create new portal Desktop and add Copied Ajax Framework Page and Custom Theme.
5) Assign this Desktop to Master Rule Collection.
You can get the details on how to do the above steps from the following Document.
Regards,
Suman