From inside a chat, tap the blue '+' icon button to the left of the text bar. Tap any picture or video you want to share, and then press send. Note that you can choose to add more pictures but pressing the big plus button at the bottom of the screen.
To send multiple photos from the beginning, tap the blue '+' icon button to the left of the text bar, then tap the photo button located at the bottom left of the screen. In this view you can select up to 10 photos, and press send.