Click "Reply" to right of the screen (located beside every post), then click "Go Advanced" below the text box, then scroll down and click "Manage Attachments". A popup box will open, now click "Browse" and find the picture you want to attach and click "Upload". Once the pictures are finished uploading you will see them appear under "Current Attachments" in the same dialog box. Now your post will include your pictures.
Or, you can use Photobucket. Photobucket is a photo hosting website. After your pictures are uploaded there you just post them on the forum by copying and pasting the provided link from Photobucket.