Anyone know how I can add my logo to SharePoint Online team site?
I just did this the other day.
For the external website, login to your O365 account, select Team Site, on the team site nav bar select Website, select your Home page, click on the existing logo on the page, and change it to your logo in the dialog that is displayed.
For the internal team site, go to Site Actions menu, select Site Settings, then select Title, description, and icon from the Look and Feel section. Under Logo URL and description enter the URL to your logo. Can be to an existing logo on your corp/personal site or you can upload it to a library in your team site (...i.e. Site Assets). If you put it in Site Assets then the URL would be /Site%20Assets/logo.gif
I can go into greater step by step detail if needed.
You need to upload your logo to change the current logo. You can see the instruction from here: http://virtualizesharepoint.com/2011/07/25/how-do-i-change-the-logo-for-sharepoint-online-site/