Have you tried absolute positioning, z-indexing and then using negative margins?
For example - the top picture should have a lower z-index than the content block below and both should be absolutely positioned within their container. This will give you a gap which needs closing so on the bottom container put something like this in your CSS:
That should then close the gap and allow the top image to overlap.
If you use 32bit PNG files you can even have transparency or semi-transparency in your image.