Replace the Default Anonymous Avatar on Blogger Comments
Step 1. Go to Dashboard - Template - click on the Edit HTML button
...click anywhere inside the code area and open the template search box by pressing the CTRL + F keys
Step 2. Type or paste this code in the search box, then hit Enter to find it:
</body>Step 3. Just above the </body> tag, add the following code:
<script src='http://code.jquery.com/jquery-latest.js'/>Step 4. Save the changes by clicking on the Save Template button
<script>
$("img[src='http://img1.blogblog.com/img/anon36.png']")
.attr('src', 'https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgcOiB2CEojt-6QYcOeGkxP1OFCFfGdnaTTPqTTUzm3jlkQObOzqmtQss-o8qLZcviR7wjDqStHSiQnSiss6GwtIwMTfBXbTfsDoyzi_ywGP5UlKZXBULwqZIct7uDvpzidZBla8c-ejg8/s1600/default_avatar.gif')
.ssyby('blank')
</script>
<script src='http://code.jquery.com/jquery-latest.js'/>
<script>
$("img[src='http://img2.blogblog.com/img/b36-rounded.png']")
.attr('src', 'https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjp0d2HKWCIRfWHqgRVz7SzMgt639LMDGZZY413K-JnwMwaTqvZlnuw2FSKja0U1DYCLE7Upivz3zLQmK2nWq9Q4fCNNT5j6m_qq8_EgFwzOVKmQkIv-VYK1rux729bG7Bj9SxWvUY1u4M/s1600/blogger-user.png')
.ssyby('blank')
</script>
Changing the default avatar
For Anonymous users: Replace the code in red with your image addressFor Blogger users: Replace the URL in blue with your own.
You can choose an avatar from here and then copy the url of it:

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgcOiB2CEojt-6QYcOeGkxP1OFCFfGdnaTTPqTTUzm3jlkQObOzqmtQss-o8qLZcviR7wjDqStHSiQnSiss6GwtIwMTfBXbTfsDoyzi_ywGP5UlKZXBULwqZIct7uDvpzidZBla8c-ejg8/s200/default_avatar.gif

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEicfvX7XGeb1eEOkHPp74EGQAFW-TOdPviN5LmLJIWcfiztDeCrRTCR6sHfyidgi1ynzgLhCGO81KTvWdnM-xueLdoa6keFxKcqUz25WsU-vUxtLqerJqWTlMZrsKCHZL1GMfcbyXj1wcE/s1600/facebook.gif

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgFUcNHTKpoTPvT_9Qeda_Ycq3HzrWEJX5EUZqfsW_x0tpzSXsT7_pK4kFHN_GCRepTIrNHaSOww7GOCk2ZYQZQqHZic4GRY139pd-_oN0-bjmaQ_CNuwso47rKMydt4i5DxL4m9oQmUlc/s1600/anonymous3.png

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEguiWdQLvvg2Sg3bcCCS5vBYfKhaVxoUMaQkidnhmnKelkgnFW-k5to_dFX62sUE6jkDMeeIZdyPgDnZW-9vpd5GNmH5wNuGVxniSrn_brrY2QFJeBIvPO-m1TRwIIV9fEftjejBq6scQCW/s1600/blogger-user.png
That's it! If you found this trick useful, please consider sharing it.