I used a platform called tinypic.com The site will prompt you to upload a photo either from your computer or your phone. Once the photo is uploaded it will give you 4 addresses or URLs, copy the last one. Go back to Steemit and paste that code in the box that is ascribed to your picture. Confirm it and your picture should pop up in a few seconds. Good luck (you can also google it and there are plenty of people showing a step by step on YouTube).