Using pics.tdiclub.com there are 3 choices, Full, Medium and Thumbnail. The 2nd 2 expand when clicked.
Here is a shortened version of the instructions.
Short version:
Log in again at pics.tdiclub.com
Select Upload button, follow prompts (check your file type)
At the end, harvest the url to use as a link for your post
Note there are 3 sizes with separate urls. For large data use the thumbnail, it shows the full size when clicked.
More complete instructions in my signature.