If your logo is not displaying on the PDF of the invoice but is displaying OK on the print preview all you need to do is install PHP-GD. Thats the GD extension for PHP, make sure its enabled - check your phpinfo.php file and all should be fine
Thanks to JBHewitt on the forum for this one!!
If this doesn't fix the issue for you then you can add the below to options to the .htaccess file
php_value allow_url_fopen on php_value allow_url_include 1
or add into your php.ini file
allow_url_fopen = on allow_url_include = 1
Your webserver needs to be able access your site - so if you have any firewall rules or Auth directives which might prevent localhost (or your webserver's public address) from accessing files, you will need to change them. For example, if you had set up Apache password protection for your site using AuthType Digest or AuthType Basic, you would need to add <b>Allow from</b> rules to let the server access the site without needing to authenticate itself:
AuthType Digest AuthDigestProvider file AuthDigestDomain / http://your.simpleinvoices.tld/ AuthUserFile /www/your.simpleinvoices.tld/.htdigest Require valid-user Allow from 127.0.0.1 Allow from 126.96.36.199 ### change this to your server's IP address Satisfy any