for posting multiple pics it's pretty straightforward. Click the + sign and then click the icon with the mountain. Add pic. Hit enter. Repeat to add another pic... If you're talking about selecting multiple photos at once and downloading them, I can't help you there.
I'm not sure how to answer getting to a specific thread. Just navigate the forum like you would on the computer. Click the button in the top left, choose forum, then subforum, etc.