"image_reduire" produces the biggest image that can be inserted in a given area.
To the opposite, "image_passe_partout" produces the smallest image
that fills the clipping area.
If you follow it with "image_recadre", with the same dimensions, it
produces thumbnails of a specific dimension, without any deformation.
For image processing with Spip, you can look here :
http://www.spip.net/en_article3476.html
.Gilles