To add a picture, the pic must be on the internet already. You can't just add it from the images on your phone or computer (that would be a nice option). If you have a pic that is not on the web, there are plenty of free image hosting websites that you can upload your image onto (you would then copy the image link of your newly uploaded image). If there is a pic that is already on the web that you want to use for your article, just right click on the image and select "copy image location" (using Firefox). Then when you're writing your article on Steemit, look for the image icon in your editor and paste your copied info in the window that pops-up. Voila!