If you are using the Launchpad Sandbox in your CAP project (like it is done here or here) and you want to change the logo in the header bar, simply add this little CSS snippet in the launchpad.html file:
<style>
#shell-header-icon {
content: url("./logo.png");
}
</style>
Of course you could also provide a base64 encoded image:
<style>
#shell-header-icon {
content: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAGGoAAAVNCAYAAAAIhfswAA...);
}
</style>