Hello @freyaastrella
Yes. I know you are using the Free version of the plugin, but as you wrote that you are a very basic web user so I have a request to report the problem to our official support where I could ask you for your access data.
It would just be faster to verify the status of your site and track down the problem that is preventing you from adding your logo to the invoice.
Regarding allow_url_fopen you need to go to the php.ini file that is on your server.
When it is disable it will have the notation allow_url_fopen = off,
while it is enable it will have the notation allow_url_fopen = on
It should be enable.
If you do not have access to the mentioned php.ini file on your server, you have to contact the company, which provides you with a server and domain, on which your shop is set up.
Best regards,