To put an image in a post you are creating. First, download the photo to your computer. And then drag the photo to where you are writing your story, and drop it inside the editor where you are writing (inside of "submit a Story").
I do not know if I explained well..
I leave you two links that can help you.
Let me know if you have any doubt!

https://steemit.com/steemit/@thecryptofiend/markdown-basics-for-beginners
https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet