i've uploaded 2 plugins with a readme.md by the same format that is specified in the docs. they both have a screenshot, and both show them at github, but non work at the forge - they both show no screenshot.
the README.md locaions:
http://github.com/arieh/ThumbsSlides/blob/master/README.md

http://github.com/arieh/MooTreeAcordion/blob/master/README.md

and the projects URL (accordingly):
http://mootools.net/forge/p/thumbsslides
http://mootools.net/forge/p/mootreeacordion