Scroll to the bottom of the thread you want to post the picture on (If you are posting a new thread skip to the next step). Then click either of the red circled buttons to get to the full post editor.
Once in the full editor, click the icon with the picture of a tree as shown in the picture below.
In the image properties box that opens, click the browse button. On a computer this will open the standard file browser to find the file. On a mobile device it will open other option that will take you to your gallery or photos. They all differ so you will have to poke around a little and figure out how to find the file you want (iPhones will have problems if you don't rename the picture prior to trying to upload it to the site It tries to use the same file name for every file it uploads and you will get an error from the forum that the file already exists).
Once you find the file click OK and then click the Upload button just below the browse button.
Click OK on the message box that comes up about being patient.
Wait for the file to upload and then you will see a preview of the image in the bottom right of the Image Properties window. Once you see the image preview click the OK button just below the preview. This will place the image into your post.
Hope that helps.