Go to Dashboard - Template - Customize - Template Designer - Advanced - Add CSS - paste the following code (and customize the code as needed) - Press enter after the last character of the last line } - Apply to Blog. 

Enter this below code the custom CSS of the blogger template.

.blogger-gear, .overview-sspe {

background-image: url('https://blogger.googleusercontent.com/img/a/AVvXsEgXYJz_NXvZ-Ahaeg49ULvBtWITPEE4ENqyZ0PMw3Kc0ymnV8C3EafXZ6mprPwN3NyZ8hHnH9205IvqiP7DXQ2PtJuICHDCYFnTzy5O-4AmSbTtS6chK1EQjSj5V8hbiXbknE4J4hmaRQUVBtpJk243SOrG-hiBbmgu0CUIB6pxmzhJyJeukq10EpUKng=s196') !important;

height: 196px !important;

width: 196px !important;

This code has been programmed to add a penguin. Replace the image to suit.