1) Insert an image from a local file (not only from a web link)
2) Be able to float the image to the right, or left, or center without needing to use BB code
3) Be able to assign a target URL for the image to link to within the same initial dialog box
4) Have a depository of images that can be re-used for inserting in subsequent posts.
These are pretty standard features. WordPress handling of images could be a good feature-set to emulate.