Hi freeko, welcome to RPoL.
What you are trying to do is not actually possible here. RPoL does not use HTML code, but its own RuBB code, which duplicates a limited subset of HTML. What you describe is not part of that subset.
The closest you can get to what you want would be to post the thumbnail, with a (text) link to the full sized image next to it.
More information on posting images and links can be found in the
Help files:
/help/?t=faqs&page=rubbcode