Resizing images will make them look very 
pixelated.  The program is choosing certain 
pixels to keep and skipping over others, so the 
resultant image will not look very good.  (Just 
take a look at the pics Yann chose for the cel 
ring, heh.)  The only time when resizing could 
work is if 1) the scanner bed is very clean, 2) 
the cel is very clean, 3) the scan is very large, 
and 4) just finding the right resize ratio.  Yann 
could make all his pics look a lot better if he 
didn't choose to resize but to resample instead.  
(I don't mean to pick on Yann, but his gallery is 
just one glaring example of this.  This is also 
caused by improper use of width and height tags 
in html, but I don't think html was the cause 
here.)

Resampling will basically redraw the image from 
scratch to a lower image size.  The image will 
then look as if you just sat farther away from 
the monitor.  Unless you resample from a much 
larger pic to a small pic, you may also need to 
sharpen the resulting image since it may look 
slightly blurry.  For my cel scans, I don't 
sharpen after I shrink the original 400-600 dpi 
scans.  However, when I make thumbnails from my 
cel scans, I do sharpen them after the resample.

Below is an example of a resize, resample, and 
resample and sharpening.  Some programs now 
automatically resample during a resize, so there 
won't be an option in these programs.  If there 
are separate resize and resample options, choose 
resample.