There are big differences in how watermark images are treated by different email clients. For example, an email client might not support body background images at all. And even if such images are supported, the email client might have fixed values for the related body background properties, such as repeat and attachment.
We recommend that you verify that the watermark image is properly rendered in each email client of interest. If you identify potential display issues, you must account for those issues in your Project design. For example, by adding textual messages to the StoryTeller template.