Header

That size isn't bad... It still fits in when viewed on a 1024px wide screen... Anything up to about 900px should be fine.
If you want to check how images look and if they fit, just use the Firesizer add-on.

I already have a banner, I just need to know on how to not make it repeat.

Could you please help me with this one Jack?

Thank you!

within #header, add :
background-repeat:no-repeat;