It's not a tab on the Edit box, the signature pic has it's own separate box underneath the Edit box and the red link to insert the pic is at top centre of that box. You don't put the url for your smiley in the Edit box, it goes in the Signature Picture box below it. If you haven't got such a box then it's a case for the Taffmeister to sort out as it'll be him that's short-changed you.
This is the link to
mine so perhaps you can see how it looks 'from the inside'. I hope you can see it and it's not just me that can. If yours doesn't look like this, you've got a problem.