After installing Ghost, I discovered that I couldn't upload images larger than 1 MB. The problem is not with Ghost itself but with a configuration setting on the reverse proxy Nginx.

To fix this, edit nginx.conf.

sudo nano /etc/nginx/nginx.conf

Add this line to the http section.

client_max_body_size 5m;

I chose 5 MB but you can choose whatever you feel is appropriate for your environment. Setting it to 0 disables the limit.

Then...

sudo service nginx stop
sudo service nginx start

Done!