Sweet, I've got images working in Bookcase. I decided to base64 encode them and stick them in the XML stream itself. That blows up the file size by 33%, I think (or is it 25%? I can never remember.) But I hope folks won't be saving 200 MB tiff images inside a Bookcase file! I'm using the md5 hash of the image data as the ID, I hope that's unique enough. I'm fairly confident that it is. Using an ID field in the entry allows me to only save an image once, even if more than one entry references it.
I got a great suggestion from a Bookcase user by email. Since KDE uses a separate class for auto-completion, I can subclass it and allow completion for fields with multiple values. Right now, it just completes to the whole string, so it depends on the order of the values. I can fix this fairly easily, I think.