I have a HTC Sensation 4G S, very similar to an iPhone 4. What you can try is...
When replying to a post, use the Go Advanced option. The forum should look like it does on a computer. There is a little paper clip above where you want to post. This should bring up another window with an option to select a photo from your phone. Use Gallery (or whatever it's called on your end) select a photo and upload. Then you click on the close window option and post. Give it a try, you may not see the photo until the post goes through, you could try it in this topic as a tester. This is what I did on my phone when I didn't have the internet. Make sure to use a photo resizing app though, as photos from cell phones can get pretty big!
***Edit to add, this worked before for me but now that I am trying to do it, it won't let me click the Choose file button :(
You could always use a photobucket account and email the pictures to it from your phone to photobucket, then copy the URL from photobucket to the site when you make a post :)
Last edited by Pitluvs; 11-17-2011 at 08:44 PM.