Clueless about coding and need help with banner

[layout_lady]

I just switched to bloggish and I like the basic style of it but would like to replace the banner headline (not sure if I am using correct terms) to a banner created in Photoshop.  It would be using part of my userpic,etc.  I've always wanted to make my journal look better but I was recently interviewed by our local paper (article not out yet) about my blog and so now I feel some pressure to make it look a little better. :)  So a few questions, what size is the banner box so I can go ahead and create the image with Photoshop I am wanting to use there?  And once I have the banner image created, how in the world do I go about overriding what is part of the standard layout?

Thanks!!