Can I use bulletproof buttons on top of bulletproof background images?

Unfortunately, these buttons can not be placed on top of background images, because Outlook doesn't support nested VML elements. In many cases though, you may be able to code the design up successfully by changing the structure.

For instance, if only part of the image needs to be behind text content, you can sometimes slice the image, use a bulletproof background image for only that part of the design, and use bulletproof buttons or inline images (held together by a table structure) for other parts.