The code generated by Flickr is HTML and not respected by our forum software which requires the use of BBcode.
When copying links from Flickr, use the second link provided--the one containing only the URL for the thumb or larger version--or copy the URL from the landing page, found in the browser address bar.
Do the following to link thumbs to original image landing page:
Replace all { and } with [ and ]
{URL=http://www.flickr.com/photos/nelsonfoto/3320783126/"}{IMG}http://farm4.static.flickr.com/3655/3320783126_0a03e54a2b_t.jpg{/IMG}{/URL}
Results in:

Do the following to link thumbs to alternate image size, image by itself:
Replace all { and } with [ and ]
{URL=http://farm4.static.flickr.com/3655/3320783126_b3da2be049_o.jpg"}{IMG}http://farm4.static.flickr.com/3655/3320783126_0a03e54a2b_t.jpg{/IMG}{/URL}
Results in:
