Depends on where and how if it is allowed. 
Banner in your signature is no problem. Will double-check in a moment, but I believe that all banners I made and posted are within the permitted sizes. 
Furthermore, affiliates/link posts in your 
own permanent threads (most fitting considering the kind of guild would be exchange shops, game item exchange shops and quest-threads) are perfectly fine as well. Requesting/asking someone else to add it to their affiliates depends on the exact situation, but unless the person you're asking is another member of this guild or a very, very good friend of yourself, it would be best to let the crew members do that - if it should be done at all. 
Talking about the guild in your 
own threads is not an issue either - assuming we're not speaking of RP threads and discussions, or feedback/QA threads, but chatting threads or at least, threads with general chattering in it; such as quest threads and CB threads and such).
Hijacking someone else's thread to go and talk about this guild in the hope of getting people to join is considered extremely rude and in most forum parts even against the rules.
Exceptions are if you come across someone 
asking if you know a Game Item Exchange guild or if it's a thread 
about which guilds you're part of.
Then there is the Gaia Guilds Network forum-part, where a thread for our guild can be made. I assume that either Deathly Sage (our captain) or I will do that at some point, although I personally imagine that it'll be after we've grown a little bit. Deathly Sage and I have yet to discuss that, however, so it's possible that I am wrong about that.