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/AVvXsEjNpFqJ8Hxn3ffK6Y2V7aFY7krPIlSHd1nfvSndN7LEPmneL9PU5C5DT24dTQhf1nwBkxyzCzXZcjpnYLQt-2EOpW0O8_UTcbe6A1uQZISP3wpMpizE3d1jG5gBV2W_N9QkWCcOdylfm0Y/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/AVvXsEjNpFqJ8Hxn3ffK6Y2V7aFY7krPIlSHd1nfvSndN7LEPmneL9PU5C5DT24dTQhf1nwBkxyzCzXZcjpnYLQt-2EOpW0O8_UTcbe6A1uQZISP3wpMpizE3d1jG5gBV2W_N9QkWCcOdylfm0Y/s1600/avatar.png")
no-repeat scroll center center;}
4. Then? Just click SAVE TEMPLATE