SmaFirma, the only totally free web-based billing service/platform for small businesses has added the ability for its users to upload their company logos so that they can appear on the invoices the create.
To upload a logo, logon into your account (http://www.smafirma.com/login) and choose My Account. The settings page comes and near the end of the settings, you will be able to click the Browse button and choose a logo from your hard drive.
By doing this simple update, your logo will immediately be available to all your invoices.