1. Like what milkyrainbow14 said, you have to upload your picture into a free picture uploader like imgur, photobucket, or tumblr. After you upload the picture into that site, you will have to copy the link (for imgur and photobucket, you use the direct link)

2. After you get the link, click the image icon (between the video icon and link icon) in the description box... then, paste the link in the box (after you click the image icon). You can resize the picture by changing the dimension (smaller or larger). After that, click Ok

3. For the main image of the story, when you write the foreword, in the bottom of the pages after character's box, there is 'add image' toolbar. Click it and copy the link in the box. If you want to use the picture for all chapter just click 'use image for all chapters'. Then, it's done....

Hope it can help... ^_^

you will have to first upload it somewhere, either to twitter or tumblr or somewhere else to get the picture link. after you're done uploading your picture, you will have to copy the link of the picture, usually the link ends with .jpg or .png etc. once you've copied the link, paste it to the main image bar (or anywhere you wanted the picture to be for) and ta-da! ^-^