There's a couple different ways you can post a picture. You can upload the picture to Flickr by using the share on your Shift. Then go to the Flickr website and go to "share" above the photo. This will give you a list of options, click on "grab the BB code" Then click BB code and your ready to copy and past to the forum. All you need is the field between [img] [/img].
An easy way to post a pic straight from your device is download photobucket from the market. You can upload the picture you want to photobucket, then go into the app. click my albums > select the photo you want > hit the menu key > click view links > all you need is the IMG link. Just click it and it will copy it to your clipboard. Then just paste in the forums! Hope that helps
Edit: BTW you can always just click on the paper clip in the forum to add a thumbnail attachment.