Is there a browse function to search for a file on your device?
If it works well with a mobile device, this feature is possibly a good enough add-on to get people to subscribe. The HTML Editor features work well on a desktop. I can't access them on my phone though, and they're always glitchy on my ipad. The result is that I really struggle to embed an image from a mobile device, especially if it's something I've uploaded to dropbox. Normally I can't make it work, and I end up having to provide a link to the photo rather than getting it to embed.
It if creates an easy way to embed images from a mobile device, it's one of the better improvements to CB that might be worth subscribing for.