Jailbreak and use installous, or got to Target and get a pre-paid iTunes card? Tapatalk is definitely worth it, it is my favorite app on my iPhone..
heres a jailbreak guide for the 4 http://forum.grasscity.com/silicon-v-alley/645249-iphone-4-0-4-1-jailbreak.html
You don't need to jailbreak just for this, get the photobucket app which can upload your pictures and create a link that you can copy and post in the forum.