1. Log in to your blogger account
2. Go to Design --> Edit HTML
3. Copy this code, and paste above ]]></b:skin>
.avatar-image-container { background: url
("https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi22GInzAUYDRGaa07lDCuIUX9sK8cM-EWCkzBXApn90jbTKwCqnpQqTVGk2uWzhUaYdE9_BWQ_kmpmkQg7T7zqNadOuZBBVoZVT88xPaME1eL9yf3DtHkmh9ECgeABz0xFbraymqVYEJI/s1600/avatar.png")
no-repeat scroll center center;}
=> the red URL is the photo, you can change it("https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi22GInzAUYDRGaa07lDCuIUX9sK8cM-EWCkzBXApn90jbTKwCqnpQqTVGk2uWzhUaYdE9_BWQ_kmpmkQg7T7zqNadOuZBBVoZVT88xPaME1eL9yf3DtHkmh9ECgeABz0xFbraymqVYEJI/s1600/avatar.png")
no-repeat scroll center center;}
4. Then? Just click SAVE TEMPLATE