Why does bliss sometimes append art and sometimes replace the existing art?

When bliss embeds art it always embeds it as a 'front cover'. If a 'front cover' already exists, bliss will replace it. If other art exists, for instance pictures of the back cover, sleeves, inserts etc, it will leave these alone to avoid losing this artwork, which may be important to you.

For file formats that only support one piece of artwork, existing artwork is always replaced.