This question seems to be pop up quite a bit so hopefully this will help out everyone out there.There are really two ways to post a picture on the forum and each have pros and cons.
Personally, I recommend and use option #2 myself. Method 1 - Forum Hosting
Below the posting box there are three tabs and one of them is 'Upload Attachment'. You can point it to a file on your hard drive, give it a comment and add the file to the post. This method will allow up to medium sized images but is not ideal if you uploading a lot of them. It also doesn't display them well, but it is one stop shop without having to deal with outside tools. Method 2 - Imgur
Before anyone posts saying there are other file sharing services out there. THEY ALL SUCK. Imgur is stable, fast and allows adult images (not officially, but they have never been known to take any down). Imgur is as simple as it comes. Upload the image by browsing your hard drive or even drag and drop it onto the page. Once uploaded they provide you with sharing links. Copy the BBCode text and paste it into your post. Presto! You're done.