First of all, you need an image and some webspace to upload it. After you've uploaded the image, you copy the link and paste it among [img] tags.
For example, let's imagine the link of your image is
https://www.myimage.com
You copy the url and write it like this
[img*]https://www.myimage.com[/img]
Remember to delete the * symbol among the []. I've only added it in order to make the tags viewable.