Signature?

I've noticed that my signature keeps disappearing...

First time it happened was using GIFs, I assumed they couldn't involve GIFs and it was altered by a moderator, second time I created my own image it was within the limits set out by the rules but now that has gone aswell, I can only assume by a moderator.

Is there a rule I'm missing to creating a signature?
Am I allowed to have a kind of trade thread aswell as an image?
Or is it not moderators at all altering it but me using links to my dropbox account?

Thanks for any help :)