How do I get pictures from my iPhone onto Facebook?

Open the Facebook App, tap on "What's on your mind" to create a new post, then Photo/Video to add Photos and videos to the post. Tap on Manage->Select More Photos To add photos to the list of Photos Facebook can access for uploads. Then Select the Photo you want to add to your post.

Alternatively go to Settings->Facebook->Photos->Edit Selected Photos to add and remove Photos Facebook has access to from your Camera roll.