Cocothumbx can create and remove thumbnails for many kinds of picture and text files. The files it supports are, according to the website:
BMP
TIFF
TARGA
GIF (animated)
PNG
JPG
PICT
PDF
EPS
ICNS
JP2
SGI
DOC
RTF
RTFD
HTML
CSS
TXT
QuickTime supported Movies.
To create or remove a thumbnail all you have to do is drag and drop the files into the window and it will deposit the file back in the place from whence it came. It works quickly and without any problems for me so far. It takes less than a minute (maybe only about 30 seconds or so) to create thumbnails for 200 pictures and that's not too shabby. There is even a little beep when it is finished.
Cocothumbx also has a large preferences menu that can be manipulated to your heart's content, although I didn't find the need to tweak anything.