First, you have to have your image hosted somewhere. Then you have to have the URL for your image.
Once you have the URL, you have to the use img tags. If you're completely clueless about this, you just need to click the little Mona Lisa button above the text box you're writing in when writing a new post. It'll generate the img tags, and you just need to paste the URL in between them.
[i can't use the real tag because it will mess everything up, but it will be in these brackets]yourimagesurl.jpg[/i can't use the real tag because it will mess everything up, but it will be in these brackets]
It should be formatted like above, except the actual tag will simply be "img" instead of "i can't use the real tag because it will mess everything up, but it will be in these brackets".