Do not need to use any hack code to magnify the image on blogger blog anymore. Now Blogger.com has provided the official Lightbox image viewer system. You can take a look at your blog. Just click on the image on your entry post, then it will magnify itself and there is also has image thumbnail under the preview image to select which image you want to look at.
Just click on the [x] button at the right top corner or just click on the dark area to back to normal.
You also can disable this feature - tutorial here.