It may be an issue with the Media Type associated with how the media comes into RootsMagic. The Media Type may be set to File instead of Image so its not being treated as an Image. It just depends on how TNG is treating it when you add the media there and then export it.
To check the Media Type open the Media Tag and look to see if there is a checkbox for marking the images as primary or including in the scrapbook. If those are not present then the Media Tag is File. When its File it will not generate the thumbnail and be treated as an Image as you want. There isn't anyway to change the Media Type once media has been added. That is on the enhancement list in our tracking system.
Since TNG supports multiple Media Types that are not supported by other software, I change the TNG export to Export As PHOTO. The import of the TNG generated GEDCOM then generated the thumbnails. So my assessment is that RootsMagic only recognizes _TYPE PHOTO
My approach was to migrating from PAF to RootsMagic through TNG so I can bring my media which I do not manage in PAF, however that leaves me with another problem to solve - How do I get my Private Notes imported into RootsMagic.
The [[ appears to be only recognized if the GEDCOM comes from RootsMagic and the ~ only if it came from a PAF GEDCOM. While writing, this it dawn on me that I might be able to edit the
1 SOUR The Next Generation of Genealogy Sitebuilding
and change it to
1 SOUR PAF when using the ~ (tilde) since my data was imported from PAF into TNG, or 1 SOUR RootsMagic when using the RootsMagic private data flag