Finder doesn't show thumbnails for Alias in OS 10.14.2
From this Apple discussion:

How the Alias-icon will look, depends on what you see in the &quot;Get
Info&quot; panel of the original file,
see the above example:
If the &quot;GetInfo&quot; icon of the original file shows the custom icon, the
alias icon also will. If the &quot;GetInfo&quot; icon of the original file only
shows a generic icon, the alias icon also only will show the generic
icon.
To recreate the proper custom icon for an image, open the image in
Preview or any other image browser, select all of the image (or any
rectangular selection) with cmd-A, and copy the selection to the
clipboard. Then paste the clipboard contents into the icon well of the
image's &quot;Get Info&quot; panel. After that also the aliases should show your
proper custom icons.