You need to host the image somewhere, like on your server, then the markdown code would be this:
![This is my image](http://www.mysite.com/assets/image-1.jpg)
Here is a real world example:
![This is my Star Wars Last Jedi Image](http://www.zeebedesigns.com/assets/image-1)
If I put the actual code in here, it will give you a poster image from Star Wars the Last Jedi, which will look like this:
Since the Forums use Markdown, it will show the image. Here is a little more explanation of the code.
The brackets  are for the Alt Text of the image, anything you type in there will be the alt text of the image.
The parenthesis () contains the URL to the image. Markdown does not allow for drag and dropping of images, if you are going to use it, you need to host the image somewhere, preferably the server where your site is. Make a folder, call it assets, images, warehouse whatever you like and put the image there. Then get the URL to that image.
I am not exactly sure what the exclamation point does ! but someone on the forums might. (found out the ! differentiates that it is an image and not a link)
Hope this helps a little.