If you're on a computer and you want to post art but don't have a deviantart (or a different art posting site) account for some reason (ex: parents won't let you) and you need to post a picture and don't want to take a picture with your phone (it looks better if you don't take a picture with your phone.) just download/export the photo (generally as either .png or .jpeg)
Then pull up Wattpad on your browser go to the create and add a chapter.
After that press the photo button in the multimedia (it being the multimedia is important) and it will pull up the files in your computer instead of the link thing.
Now just locate where you put the picture and press open (I think it's "open" I haven't checked so I could be wrong.)