Yes, you can include logos within your mailings and on your envelopes.
You can either include logos in your supplied document template or you can have a range or pre-selectable letterheads in your Docmail library.
Docmail also gives you the option to design your own custom envelopes, in which you can upload an image (such as a logo) and text to the outer envelope.
Docmail can also process images, so these can be included in your mailings.