Ah assuming you mean it's ok to post the image, description, and link to FBTB, but other than that creating advertisements for it is the issue.
For example this is what I have posted in my image description on Flickr. Is this ok?:
"Version II is updated by refining the flame technique and giving Jean a pair of yellow cloves in which to asp!
Entry for FBTB Heros vs. Villians Mech Contest:http://www.fbtb.net
Or do you think no link to FBTB.net is preferred?
It would be nice to reference the contest requirements and to "advertise" to get others to join the contest but I can see how even just a "FBTB.net" link could taken as a "come vote for me" advertisement.