When you go to User CP, look under settings and options. There you will see edit avatar. There is where you will upload a picture for your avatar. I use Photoscape to make mine animated.
If it doesn't load it might be too large.
Posting pictures in a thread you will click on the paper clip above. Then click on manage attachments. Then again, you will upload your pictures to post.
I don't know about posting directly from iPhone. I always download mine on my computer. Maybe someone else will know how to do it from the phone.
Hope this helps!