Since September Google has allowed users to use customize Google Forms by inserting images and logos into Forms headers. Now, as I just learned from The Next Web, you can insert images and logos into the header of a Google Sheet. To do this just open your header cell, select "image" from the "insert" menu, and then upload you image. You can re-size your image by simply dragging the corners. Click here to see Google's GIF of the process.
Applications for Education
This update isn't going to change the way that you use spreadsheets, but it could be a nice way to brand a report that you plan to share with others.