A quick test suggests that it does not so work though there is no info in FileMaker help to suggest that this is the case.
I set up an unstored calculation field to return getThumbnail, with container specified as the result type.
If I use insert picture to insert an image with "store a reference" specified, I get a question mark.
If I drag and drop the same file into the container field--embedding it, I get a thumbnail image in the calculation field.
I recommend that you report this in Report an Issue as a bug.