Click Start Writing from any prompt to start your story. From the Add Story page, click Add Media:
This will take you to your Media Library. Click Upload Files.
Either drag or drop an image from your computer into this window, or click Select Files, navigate through your folders, and select the image you want.
If there are other images you want to use in your story, click Upload Files again and repeat the process. Now they are in your Media Library, ready to be added to a story.
Click the X in the upper right corner to go back to the Add Story page.