1) Upload your own image directly
You can upload your own image by pressing the My Media icon, then select Upload and pick your image. Touch the preview of the image you want to upload and press Select.
ALTERNATIVE METHOD - 2) Paste an image URL in to your discussion post
Alternatively, you can upload your own image to https://postimages.org and then copy the 'Direct Link' URL.
You can usually find the URL of an image by right clicking on it and choosing copy image address, or open image in new tab and it will be displayed in the address bar.
That's it! When you submit your post it will automatically convert your url text in to an image, just like this: