You *might* be able to do something like this:
image.whatever {
margin-bottom: -20%; /* Adjust as needed */
max-width: 100%;
}
image.spacer {
max-width: 50%;
float: right;
}
And make the spacer image be the same dimensions as the bottom tail of the image, but transparent.
Be aware, however, that this borders on insane, and that CSS really wasn't designed for that. CSS3's exclusions will make this better if anyone actually gets around to fully supporting them.