Ah. This is what I wanted this morning before I was thwarted by Google.
Tony diTerlizzi's interpretation of slaad.
This chap is both hilarious and obviously dangerous. It was partly his illustrations that made me fall in love with Sigil and Planescape in the first place, so now when I think of slaads, I think of this.
And a swift edit to add another superb image of slaadi - thanks, Chris :)