The first thing to check is your e-mail client's settings. Many e-mail clients do not show images by default. However, they will all have an option to enable images. Unfortunately, there is not anyway you can prevent this issue from happening.
If images are displaying correctly, then the issue may be that your CSS is not being applied to the e-mail. Some web-based e-mail clients (ie. Gmail and Yahoo) do not support external CSS files. Therefore, when someone views your e-mail in one of these clients then the CSS will not be applied to the e-mail.
As a workaround to this issue we suggest that you insert the CSS styling inline into the Page Style that is being used by the e-mail that you are sending out. Please see example below.
eg. <div id="example" style="border: 1px solid #fff"></div>
In order to prevent this from happening do not include a menu region in any Page Styles that you are using for the Pages you are sending in Campaigns.