First, create a video or photo to your Story like normal. Then, click on the sticker button (the one that looks like a square smiley face with one corner pulling up). Slide down until you see a camera-roll button, which features a tiny circular thumbnail of the last picture in your camera roll. From there, select a picture to add. You can adjust the size with your fingers, then adjust the size of the original photo or video you took. Add as many more photos as you want, arranging them in whatever size and position you want with your fingers.