You can customize modals visually with CSS.
For example
.d-modal.login-modal {
background: url(https://upload.wikimedia.org/wikipedia/commons/9/96/Ambrosius_Bosschaert_the_Elder_%28Dutch_-_Flower_Still_Life_-_Google_Art_Project.jpg) 0% 0% / contain;
}
or
.login-modal.in .modal-inner-container {
background: url(https://vignette.wikia.nocookie.net/mrmen/images/b/b7/MMS_MrTall_ca03-1-.jpg/revision/latest?cb=20101106151544);
background-size: contain;
background-repeat: no-repeat;
background-color: #fff;
padding-left: 140px;
}
This isn’t functionality that comes with Discourse, so this would require a plugin or theme. There are some similar howto topics that could help, like https://meta.discourse.org/t/banner-themes-and-instructions-for-customizing-them/82368 — there’s an example there that helps you create a collapsable banner. Dismissing the banner entirely wouldn’t be a vastly different process.