You have some options available. They all are inside the “Share” popup appearing when you click the “Share” button next to the Like and Fave buttons:
The BBCode for forums appears when you click the last “link” (I should probably make that a button!) and the code is automatically copied to the clipboard.
If you’d only like to get only the URL for the thumbnail, as a workaround you can extract it from the generated BBCode - just look for the [IMG] tag:
In fact you can also extract the URL to the picture page too. You can also get an HTML link from that Share popup if entering HTML is an option available in your forums.
Does this work for you? I guess I could add the option to have a bigger thumbnail if needed, let me know.