Do you have cpanel or window server.
If you have cpanel login in you come all the icons like desktop on your computer.
Click on file manager
there may be popup select home directory
click on public_html folder
If you installed in root that is your domain name then you should see wordpress folders and files
you will see a folder where you installed wordpress or any other name you gave when installing click on folder and you will see the wordpess files and folders
click wp-content folder
click themes folder
click on wooden-mannequin
click on img folder
in there you will see front.jpg replace this file keeping the same name.
Widow server is different and have not worked on one.