There are two ways an image can be stored in a container field:
- The actual image is stored in the field.
- "Store by reference" was selected when the file was inserted. With store by reference a filepath to the actual image file is stored instead of the actual file.
Sounds like option 2 was used to insert the images. Since your email recipient doesn't have access to the original files, the container fields don't display any images. You'll need to either send a PDF of your layout or re-insert the images so that "Store by reference" is not selected.