Just follow these simple steps.
Step 1. Go to your dashboard >> layout >> Edit HTML >> Expand Widget Templates
Step 2. Search for </head>
Step 3. Add following code just before </head> tag.
Step 4. Replace <body> with <body onload='setupZoom()'>
Step 5. Save your template.
Step 6. Now that you have added the code for Image Zoom Effect to your template, you must add following code while addin any image to your post.
<a href="Image address"><img src="Image address" width="200" border="0" height="200" /></a>
Here, "Image Address" is the url of the image you want to add.
See Demo here.