Ok, so you have a great picture you want to share with everyone. You've cropped, sized, and rotated it to be exactly what you want... Now all you need to do is get it online!
First, you need a place to host your image. If you have a personal server, just move the image onto it, and you're done. Otherwise, do a search for image hosts, you'll find a lot. I use PhotoBucket, but only because I found them once on accident & just stuck with them.
Be sure that your image host allows hotlinking. If they don't, people won't be able to see your image in the forum...
Ok, once you've found a host, get an account, and upload the image. Each host does things slightly differently, so follow their instructions. Usually after uploading the host gives you an address or two: one that is the direct link to the image, and one to the gallery page that it is viewable on.
You want to use the direct link (often referred to as URL, it should end in "gif" "jpg" or another image type, NOT "htm" "html" or "php") when posting.
If you're with a nicer host, they will also give you a tag (to use in html pages) and a BBCode (for use on bulletin boards and forums). If they do, your work is done, just post the BBCode directly into your message.
If all you have is an address, you have a little more work to do. Copy the image URL, return to your forum, and open a new post. Type your message, then get ready to insert the image.
Click the button that says "Img". It will change to have an asterisk in it, indicating an open code, and you will see the following inserted in your message:
[img]
Your cursor should be right after the "img" code, so paste the image address in, then press the "Img" button again, which will close the code. That's it! You're done! Click "Preview" to make sure it worked right, then finish your post...
Ok, example time. I want to post an image. I've uploaded it to my host, and it didn't give me the BBCode, just an address (PhotoBucket does give BBCode, but for this example pretend they didn't).
The image is located here:
http://i40.photobucket.com/albums/e245/djpsychochild/Image006.jpg which means I need the following code to get it visible:
[img]http://i40.photobucket.com/albums/e245/djpsychochild/Image006.jpg[/img]
Putting that code into my post will result in this:

Tada! If you have any questions, feel free to e-mail me. If something isn't clear, PM me and I will try to clarify it...