Since upgrading to 10.14.2, the Finder doesn't make thumbnails for aliases of jpeg image files, instead, it only displays the generic JPEG icon. Is this a new bug, or is there a hidden setting somewhere that would correct this?
How the Alias-icon will look, depends on what you see in the "Get Info" panel of the original file,
see the above example:
If the "GetInfo" icon of the original file shows the custom icon, the alias icon also will. If the "GetInfo" 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 "Get Info" panel. After that also the aliases should show your proper custom icons.