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/AVvXsEgTeuOvZDNk-CjtV7v_6J-7RG7AOyWqVwgVmMkwKVcuemoYwxP4QtMjSMAJBBbzsCQQCYmICma9RrvuLwXOZ8eI0HkJc31qHRCwE9a-D0ApTBUFdKfhSaQSvTcIJbIdDsRCQ3spRi-WZ0g/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/AVvXsEhns3ld62MPK4YA0zibGAhCiUj3iofg8acvzKFVzvNwU45ay_PTea2mcSKt5maqcbo86HmU4pQP8NOm2T73WHTNMAWpmVQBqqL1V9p2GBgRgldT3t62Xl5oRp1dvQMcKG08NG76a3yDfGg/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/AVvXsEgTeuOvZDNk-CjtV7v_6J-7RG7AOyWqVwgVmMkwKVcuemoYwxP4QtMjSMAJBBbzsCQQCYmICma9RrvuLwXOZ8eI0HkJc31qHRCwE9a-D0ApTBUFdKfhSaQSvTcIJbIdDsRCQ3spRi-WZ0g/s200/default_avatar.gif

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhJtmgR530SnE0FvB5nP8F9sVYerccc0X-QvX4sgWW9I7r9scN3mhEw8OmIsZATlEzcs4WINJArBNBqTjiwI6lmJ8e2BUIM2RxhOgAtE6IQfJwEpcRKlWhK7q2WDQQsrk8UNgak-BzHPJs/s1600/facebook.gif

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiSRiF0G8TByJeilJWTZXrik0Q6f1scliKaGHQfF0V2W86AomRgO_QRIzVkjYYJjISu1sMlsoHK45TiHXc-tbw8hxaEd23YixloWsvePTINr4SRrdZs4o9ZG4wJa5gkJ8rvCu5FRV0uQEo/s1600/anonymous3.png

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjGxa9ttEyAPv7CaB2P-GdUl-dZ6_3c9-co1j9MGVpSOesp_G7-Lu5H0fSvRFo6WVgQTCYRrqzccI9lFsdLyGyIxMzaTHSzCpduqRP4JJwX3vSvd8kK-ePWmV_WvdA2j_6RxYqbfwOAjmeC/s1600/blogger-user.png
That's it! If you found this trick useful, please consider sharing it.