Make sure you're using the hostname setup when you installed. NGINX (which the install.debian script on github uses) will reject requests with an HTTP referrer that doesn't match the hostname, such as if you use the IP address.
I installed it on Ubuntu 12.02 in a VM, and could not get images to display until I figured this out. I eventually removed the following bit from /opt/nginx/conf/sites/danbooru.conf:
valid_referers none __hostname__;
if ($invalid_referer) {
return 403;
}