You can always upload and add your own logo or background images to surveys.
Click on [Logo] on Themes page to upload or change logo. Your logo will appear on the top left corner of survey. If it's big, try adjusting height under Logo section. But if it's too big, we'd recommend you to resize the logo image first and upload again.
As your respondents' screen resolution varies, the width of your uploaded background image should be at least 1440px so it can cover most screens.