First you go to SteemImg where you can host your images for free. My library which is creative commons zero is where you could start practicing:
https://www.steemimg.com/steemitqueen
Ok, then, on the "Embed Codes" tab, you scroll down to the "Markdown" one and hit copy. Then paste the code into a comment or post.
Here's the code: 
My library has about 260 images which are free for anyone to use for any purpose. You can also use unsplash.com which is creative commons zero too. Then you need to upload those to SteemImg, starting your own account or using it as a guest. Steemit doesn't host images yet, so you must use the Markdown code.
now you can just copy and paste an image or its URL into the coomment box and boom there it is