There isn't a setting for this. The only way to change that is to either 1) alter the login_form
admin template, changing {{ base_url('styles/default/xenforo/xenforo-logo.svg') }}
to direct to a different logo image or 2) uploading a new image in place of xenforo-logo.svg in that location.
Worth noting that, according to your cache rules, simply replacing the image may not make an immediate difference.
Oh yeah...and, if you go with option 1, you may also want to change the width and height attributes in that same <a> tag.
Disclaimer: Please be aware that, because this is in admin template, editing it can only be done in the Master style. This means that once you make changes to it, there is no simply reverting back to the original. Also, if you mess something up in this template, it could prevent you from logging in to your admin panel. If you're not absolutely certain about what you're doing, I highly recommend a backup and testing what you're wanting to do in a test install before you do it on your production site.