There are two ways.
1) Click on the paper clip icon above the space where you type in a message, then upload. You then choose which picture (if multiple uploaded) you want to place at the cursor location by clicking on the photo from the little pulldown menu next to the paperclip icon.
2) Easier way, scroll down below the window where you type in a message and click on "Manage Attachments". Find the photo file and upload it. The photo will show at the bottom of the post.