Sorry you're having such a problem...use the original portion of the instructions:
The previous instructions were:
When you're posting a thread and want to add a thumbnail, write what you want, then scroll down and you'll see a link to Manage Attachments (you have to be in the Advanced section). Browse your computer for the resized photo you want, click upload, close that window and submit your thread.
Let me know if that works for you...it should. That's the way I do them