Default Display Order

The default display order lets you choose how results should be ordered when no real query or sort is requested. This is typically used to promote your most relevant data to the top of the page.

By default, results are sorted by

_docid_ desc

this will show the most recently indexed items.

Alternatively, you could apply a boot query to elevate Raster results and items that have thumbnails:
query({!lucene v='fb_hasThumb:true subtype:Raster'})