1. Click on the picture in the post. This brings you to the picture in Picasa web.
2. Click on the "Download" menu and choose "Download Picture" (exact wording may vary).
That's it, you're done, you now have the exact picture I uploaded.
This applies only to pictures in posts starting from today. Please comment if this does not work for you, or if you find it too cumbersome, or if you have a better method.