This discussion belongs in Off topic.
You first need to upload them to a webspace. You know, like uploading websites.
After that, you need the url of the picture. To get it, right click on the pic, select properties, and have a looke at the info. Somewhere should be: URL : [url="http://[url]"]http://
Copy the url.
Then, go visit the forums and use the Image UBB code to show it. That code is:
(Remove the asterisks = *)
[img*]URL[/i*mg]
At the place of URL, you copy the picture URL. NOTE: The http:// part is REQUIRED!
For example, here you see the URL of a pic of a Rayman fansite:
http://www.rayman.xnapster.com/magician2.gif
And now, I'm going use the code:
Nice pic, not?