First the image has to be stored on a server.
Option 1) There are forums that allow you to upload your image to our servers and then link to it there. Simply click on the "Attachments and other options" link at the bottom of the edit box, where you create your post. If the option to store the image is available in that forum, then you will see an "Attach:" option including a "Browse" button.
Option 2) Use a third party to host the image and upload it there.
Now that you have the image stored on a server. You need to grab a link to it.
If you used "Option 1" the link will be available AFTER you make your post. It will show as an attachment in your post. Right click on it and save the link location/URL.
If you used "Option 2" above, then you need to copy the link location from the site hosting the image.
Now that you are armed with the link, edit your post and click then
button (first button in the second row of buttons of the edit box).
You will note the editor places image start and end tags where your cursor was located in the edit box. Between those two tags, paste your link into that. When done, you can "Preview" the post and the image, if all went according to plan, will appear where you placed it in the post. If it all looks good, then hit the "Post" button.
Hope that helps.