Change image on Hover / Making Rollover image Effectin Blogger

<a href="URL ADDRESS"><img src="URL OF THE FIRST IMAGE GOES HERE" onmouseover="this.src='URL OF THE SECOND IMAGE GOES HERE'" onmouseout="this.src='URL OF THE FIRST IMAGE GOES HERE'" /></a>

<img src=""
Here Paste the URL of First Image.

onmouseover="this.src=
Here Paste the URL of Second Image. Which will appear on Hover.

onmouseout="this.src=
Here Paste the URL of First Image.

Now you can paste your image inside a blog gadget, going to Layout > click on Add a Gadget link (right side) > Select HTML/JavaScript from the pop-up window, then add it to your sidebar.
You can also add it inside your post by going to New Post > Switch to HTML tab and then paste the code in the empty box.