Are you using the IPF app? When writing a response in the new reply box, in the top right next to send is a button called advanced select that then upload from camera or photos. But you must have the iPad forums app from AppStore.
If its another forum you may have to find an app for that forum, failing that upload the picture to photobucket (I'm sure there's an app) then each picture you uploaded has a link which you can post onto the website. Good luck.