These are general guidelines/recommendations for creating a custom theme background/banner for your site.
• Background Image Size: 1600x1100px
• Background Image Format: JPG is the best option to keep the file size down. However, you will want o use PNG for transparency.
• Banner size: Your width should match your background image size. You will want to create your banner height based on where the top of your horizontal menu will start.
• Banner format: Transparent PNG is the best option.