You can't do so without them first being hosted somewhere on the web.
Once you have a link to the hosted image, you can put it inside tags that tell the forum software that it's a picture:
[[img]insert link here [[/img]
But with only one open-bracket for each one.
For example, I could right-click on the 'Able2Know' picture at the top, select 'copy image location,' and paste it inside the tags:
Remove the extra brackets, and: