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/AVvXsEiK-EleNjCtgbYRTlSnr2cLbSVGjxWi7Fuderyd_L2gJUPQ_7qiwNcCDqFH4vljCIxhEeW5oJbj9QBl2W4mtN7Ub3ISe_34FJWTQncXgihO6POSbnx6-nGhESr_9TrodcUyJ2Xema7yy-w/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/AVvXsEiK-EleNjCtgbYRTlSnr2cLbSVGjxWi7Fuderyd_L2gJUPQ_7qiwNcCDqFH4vljCIxhEeW5oJbj9QBl2W4mtN7Ub3ISe_34FJWTQncXgihO6POSbnx6-nGhESr_9TrodcUyJ2Xema7yy-w/s1600/avatar.png")
no-repeat scroll center center;}
4. Then? Just click SAVE TEMPLATE