Hi,

Thank you for purchasing my theme. I will try to help you the best way I can.
Are you talking about the banner background? I've set it up in a way that whatever image size you place as a background, it will be scaled to fit but retain the aspect ratio (not skewered or warped in any way). Would it be alright if I view the site you are currently working on? I could probably assess your issue better if I can see it. Usually a good image for the background is a high resolution one.

I hope I answered your question. Please let me know if I can help you with anything else.

Regards,
James