With what did you upload the picture?
Imageshack or Photobucket?
you have to take the "Hotlink for Forums (1)"
And with photobucket
you take the "IMG code"
just copy and paste it (User Control Panel --> Profile --> Edit signature --> AND (that happened to me often) don´t forget to "Submit" the signature)
Those options below the field, where you can edit your siggy, should NOT be ticked.
Then it should work.
I hope I could help you. If not, just tell me.