What? You mean so that you can use it on these forums? What I do is save it with the black background, open it in Photoshop, delete the black background using the magic wand tool and then set the background layer's color to that which is the color of the forum.
The color of the signature background on these forums is:
Hex: 0d253e
or R:13 G:37 B:62
I'm not good at explaining things, but you hopefully get the grasp of what I'm saying. Good luck. Hopefully tomorrow you can let me know the actual problem and I can help you some more . You can always Google your problem and I'm sure you'll find the answer.
I'm not a graphics expert but I've helped you as much as I can.