When you post a message, below and to the left of the text entry box you will see +Additional Options. Click on Additional Options and some "Additional Options" will appear. One of those options is Attach: You will notice the Browse button to the right of the text box. Click on the Browse button and it will allow you to select a file on your computer to include in your post. If you want to add additional pictures or files, click where it says (more attachments) and another text box and Browse button will appear.
IMPORTANT: There is a limit on the file size you can attach. No one file, nor the combination of all files can exceed 196 KB. If the sum of your attachments exceed 196 KB when you go to post your message, you will lose everything. If I am doing a post and I'm not sure if I will exceed the file size limit, I will copy my text entry before hitting the Post button. That way if the file size is to large, I can start a new post and paste in the copied text so I don't have to start over.
If your picture files are too large, you will need to re-size them. You can generally do this in photo editing software. I use Photoshop Elements There is a menu option for Image/Re-size. I change the resolution from 180 to 72 and then save as a medium quality Jpg file and that usually does the trick. Another thing you can do is to crop the picture to include only what you want to include. That will cut down on the file size as well.
While many forums require you to post your pictures on a picture hosting site, this one does not. Personally I don't like the pictures posted on a hosting site because when those pictures eventually get deleted, they also disappear from this site. Then all we will see is little boxes with red Xs.