The hosted site campaign banner is 1050 pixels wide by 262 pixels tall. (Hosted Site Appearance > Banner Image).
The integrated campaign banner is 1050 pixels wide by 175 pixels tall. (Integrated Appearance > Banner Image).
We recommend designing any graphics with these dimensions from the start, but don't worry if the image you want to use is not those specific dimensions. You can upload an image that is different dimensions than this, and then click the edit (pencil) icon to crop into the image.
Here is what the integrated campaign banner looks like once published:
You can also use banners to stylize your campaign title and include other information. You'll want to design the image with text in it before you upload the banner. Here are some examples:
If you don't have someone that can design a banner for you, don't worry. Your Success Team can create one for you as a standard part of your campaign creation.