Here is a tutorial how can you add thumbnail on forum:
After entering this page there's a button "Select" on the right. Click it.
There will be a pop up where we have to select our picture we want to upload. After selecting your picture click "Open".
Next press "host it!".
Now wait till your picture will be uploaded to the server. After a while a page with some links will open. You have to select a link which will make your picture visible as a thumbnail.
You copy it and paste to your post. That's the result.
