When using merge tags it is impossible to set URLs as placeholders for previews. Because of that all images that have src="{merge_tag}" are not shown in previews which sometimes significantly changes the overall look of your email.
There are two solutions:
a) Allow using URLs as placeholders for previews of merge tags. As your team explained it seems to be impossible because of security reasons
b) When a merge tag is added as an image source add some picture/icon to its preview to make it clear that there supposed to be a picture in this place.