The forum has file size limits on photos that you can attach to posts, and your phone's default setting is probably way over that. (If we didn't have size limits, we'd run out of server space pretty fast and have to keep buying more. )
The workaround: upload your pics to an image hosting service (like Photobucket), and then grab the link to the image to post here.
You use your phone just like you use a computer. On mine, when I get the "browse" button to upload a pic, my phone asks where I want to look, and one option is "gallery" where all my photos are. I just choose from there, and upload works fine.