hello Michelle Ribeiro,

Unfortunately that was not possible to do it.
Unless you add this css code in your template css file :

body div#fd .mt-20 {

background-image: url('/templates/protostar/images/login.jpg');

background-repeat: no-repeat;

background-color: #e2e2e2;

}

The image path URL is follow what your image stored in your site.

By the way, we have "EasySocial Quick Registration" module including this feature.