Don't use 80, 81, 443, or 8080, those are common ports that webservers are run on.
I wouldn't run it on something obvious, like 21, 1337, 31337, or, anything under 1023 (the services port range), really.
I ran mine on 1024, but once you get over 10,000 noone really bothers scanning that high. (unless it happens to be a known trojan port). However, the higher it is, the more painful it is to tell people in URLs (if it's just for image hosting, then it's not that big of a deal).
Easy to remember ports are better, like 11111 22222 33333 44444 55555, 12345, etc