Following on from the tip on how to take a screenshot, this post explains how to attach the screenshot(s) to your forum post.
For single screenshots, the easiest way is to type your post (either in the Reply box at the end of an existing thread, or into the Post Thread box when starting a new thread), then drag/drop your screenshot into the message field.
Alternatively, click on the "Attach files" button. You will find this button immediately below whichever box you are typing into, see this image of the Reply box:
This button will bring up a file browser, which you use to locate and select the image that you wish to attach. After you've made the selection, and have finished typing your post, click on either the "Create Thread" button (for new threads) or the "Post Reply" button (when posting to an existing thread). Your post will then appear with the attachment shown as a thumbnail, though viewers can view the image at full size by clicking on the thumbnail. You can add multiple image attachments to a single post using the same process, and they will all subsequently appear as thumbnails shown side-by-side at the bottom of your post.
You'll also notice that when you use the "Attach files" option, a thumbnail of the selected image will also appear in the "Attached Files" list immediately below the box that you are typing into:
For single images, you can usually ignore this thumbnail and its options. However, should you wish to include multiple image attachments, and you wish to include them "inline" (i.e. between blocks of text, as the attachments in this post are listed), then you will need to refer to these thumbnails. To insert an image "inline", hit enter to move to a new line, then use the "Upload a File" option to select the image. When it appears below the typing box, click on the "Thumbnail" button immediately below it (it's suggested that you don't need to use the "Full Image" button, as that will help other readers when scrolling through the thread). This will insert the thumbnail into the text box at the point of the existing cursor position, with the cursor moved to the end of the thumbnail. Hit enter again to move down to a new line, and carry on typing. Use the same procedure to add more attachments "inline", until you're ready to submit your post.