NationBuilder donation pages include an intro text section where you can post a personal appeal or upload and post an image. It's a free-form text field like this one, so you have a lot of flexibility. To access that section of your donation page in your control panel, go to Websites > {page name} > Donation settings > Intro. (You can find it easily with the the page-type dropdown menu if you have a lot of pages.) You can then edit the intro text for your donation page.

If you need to add a custom disclaimer that you want people to check off before donating, you can add your campaign finance disclaimer under Settings > Bank accounts by editing your bank account settings.