Hi Bastian,
You can include images! - In a Lotus Notes signature you can import a signature with an image via the "Import Rich text" button. This will translate the Rich Text into DXL (sort of html) containing the text, text format and the image. The picture will be translated as a section (can take a while to import/format).
When the signature is pushed out to the user, the DXL is translated back (imported) as RichText, including the image.
If you want to include images in the html signature you should construct a html
tag pointing to an image on a website accessible by the iNotes client. Note however that anti-spam solutions issue 'spam points' for html messages that include image references.
Duco