I think there's no easy solution for such a "simple" job, but anyway you can
give a try to:

-use a depth of 1 from the page containing the thumbs (assuming images are
there, one step ahead from that page)
-never forget to use --near option
-use a different local structure: maybe the one you need is given with option
-N4, where you will end up with several subfolders, each one containing a file
type (gifs into gif folder and so on). Bear in mind you'll have thumbnails too
into those folders.

There's no easy way to rip thumbnails because there's no exact size limit for
them: e.g., on one site thumbnails could range 15-25kb, while in others this
range may include some final images too.

Hope this helps (a bit ;)