You need to be in your profile first, then go to settings, then on the right side of the page you will see signature, click on that, there should be an option to upload from there, you might have to delete your old sig first, not sure about that though, i just looked in my settings and i think things have changed since i put my last sig in, although it was only about 3 weeks ago.
It has changed a little with an update of IP.Board. If you go to change your signature and the toolbar is grey-ed out you need to uncheck the box and possibly toggle the mode after that with the control on the top left of the toolbar.
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I go in my profile click on the signator but there is not box there to check maybe I'm blind and my signator is there twice.