Yes, you can if you are on our Professional plan. Only an architect can do this however.
Simply go to Settings > Configure domain, here you can upload an image/logo for the login page (max height 152px) and for the top left corner of the header bar (max height 50px) of your domain.
Transparent (i.e. no background) PNG files often work best.
Click the upload button for each and when you're happy click the tick.