For future reference, if you have questions like these you should feel free to PM any of the ATT moderators - we'll be happy to point you in the right direction.
To answer your questions, yes you are more than welcome to create a thread for your Tau army - we encourage it in fact. The appropriate sub-forum would be either the Common Workbench
or the Project Logs
. Do note however that like all sections of the forum the ATT rules and guidelines are still in full effect on these sub-forums.
There is no character limit on ATT. There is however a limit of 25 images if you wish to upload your images directly to the site. If you want to post more pictures, you'll have to link to them from somewhere off-site. Photobucket used to be the default recommendation, but after their recent policy change this is no longer feasible for many. A quick search should bring up a list of alternative image hosting sites.