Can I have an example of how to submit Synfig work into the Forums?

If you want to include a picture, you need to host it on a web host like your ISP webpages or a photo service like flickr or photobucket.
Then include the line

[img] _picture.png[/img]
[/code] in your comment

If you just want a link to a sif file or non-image file, use [code]
[url=]Friendly name[/url]

See also faq.php#24 which basically says the same thing…