How do you make part of your sig transparent?

I tried to color it the same as the forum page, not working. I tried to erase the background till it shows the checkered default background (in photoshop) and still, when I posted it, it shows a different color

Thanks in advance

EDIT: Look at my signature to see what I'm talking about
 
when it shows the checkerboard, save it as .png. Its the only way to make it lagit transparent. Except .tga, but thats only really for ingame sprays.
 
I'm using photoshop CS3

Now that I look at it, there is a small but transparent area on the bottom right corner of the sig, it's like a horizontal line running from the corner to the middle of the sig
 
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