Adding your own photo to a post is really straightforward. Firstly, just make sure you have the image you want to use saved somewhere on your computer. Next, when you're in either Create Post or View Post, scroll down to the bottom and you will see there is an add media section (if you don't see it there, then it means there is no image on this post type).
Here you'll see the section where you can either drag and drop or select a file to upload. If you click this button, it will open the folders on your computer, so you can locate the one you want to use.
Once you've found it, select it and click 'open', this will add the image in to your preview pane on the right.
Last step is to ensure your image is looking great. If you need to, you can click on the image and drag to reposition.