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', 'http://1.bp.blogspot.com/-Zphr2YJH_6w/T6ZZE4YeNBI/AAAAAAAACF0/Tyuj8hkOpdc/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', 'http://1.bp.blogspot.com/-eKbzORzVaBQ/T6ZXHmdgHqI/AAAAAAAACFs/rVy3T4gxojM/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:

http://1.bp.blogspot.com/-Zphr2YJH_6w/T6ZZE4YeNBI/AAAAAAAACF0/Tyuj8hkOpdc/s200/default_avatar.gif

http://3.bp.blogspot.com/-p4JvM7rWNG4/T6ZcU5eKqTI/AAAAAAAACGM/K0DB35A5brE/s1600/facebook.gif

http://2.bp.blogspot.com/-5nGzg5T-9qA/T6ZbL9JF0iI/AAAAAAAACF8/TvTnURwsNb0/s1600/anonymous3.png

http://3.bp.blogspot.com/-oivPlkvBNBY/T6ZoNQhfAII/AAAAAAAACGY/3gem5DBmmQ8/s1600/blogger-user.png
That's it! If you found this trick useful, please consider sharing it.