Right... I've just enabled the following for all Users:
Can Upload Images for Signature
Can Upload Animated GIF for Signature
Signature Image Maximum Width (pixels) 500
Signature Image Maximum Height (pixels) 100
Signature Image Maximum Filesize (bytes) 10000
With signature images, you can point to a URL or you can upload from your computer.
I've tried copying and pasting your signature URL and it didn't appear. I think that's because its dimensions don't comply with 500 x 100 pixels (its height is 140).
I noticed that you have the same bunny pic as your profile pic. I think it's not appearing there because your profile pic needs to be a maximum of 100 x 100 pixels. I also noticed you haven't uploaded an Avatar pic yet.
If you look at Jonesy's profile, he has all three: Lovemate pic as Avatar, muppet pic as Profile pic and text as signature.
Have a little play around with your 3 image options, making sure you comply with the size restrictions, and see how you get on. You should be able to upload pics from the Edit Profile section of Forum. Any probs, give me a shout.