If the picture is already online, just include the direct link to the picture as text. You can get it usually by right clicking on the photo and selecting "copy image URL".

If it's not on the internet, you need to host it online with a site such as imgur.com

Here's more info: https://steemit.com/picture/@bismail/how-to-upload-and-embed-a-picture-to-your-steemit-blog-posts