Help With Banner ;d?

[commonsy]

Right guys basically this is my forum - http://www.thefifaforum.co.uk
And in my view it looks like this

But if i viewed it on another pc the length of the banner changed! it either stretches or dont fit! CHECK IT OUT YOUR SELF - http://www.thefifaforum.co.uk

Is there a way that the forum will always stay the same size so i dont have to alter the banner? or that i could keep my images on the left i made then repeat an image on the right?

Please help me

Thanks
Daniel Commons