HELP, question for making sigs with paint

[uzumaki chris]

when you get the sig backround, for example

You must be registered for see images

and you get the anime render, for example my avatar. how do you add the render to the background without adding the white part by using Paint software

 

[Phobos]

which white part?
well anyway i don't have paint and every1 says it blows
get PS or at least a GIMP(free)

 

[Wildtangent]

which white part?
well anyway i don't have paint and every1 says it blows
get PS or at least a GIMP(free)

I use gimp it works nicely but is kinda confusing.