OK, I can't say I know what the problem is, but here's some observations:
I have looked at 3 such signature pictures - Tony Draper, Juud and ANO.
1) They ALL refer to items hosted on photobucket - so if you can see one...
2) 2 are GIFs, one is JPG - the only one not appearing is TD's GIF.
3) It is possible to view TD's GIF directly in photobucket.
4) The behaviour is identical in both FF and IE6 - although in FF the missing picture is simply not shown, rather than the placeholder x in IE6.
I conclude that there is nothing in PPRuNe or photobucket to prevent such signatures appearing.
However, there may well be something to do with TD's PPRuNe account of which I am unaware.
SD