SPIP has a library of graphical filters that are used by default to easily help resize images. These functions are defined in detail in ecrire/inc/filtres_images_mini.php. The "Filtres Images et Couleurs" extension, which is active by default, offers numerous other filters as well, like creating text images or using masks, merging images, colour manipulation, etc.
In order to avoid recalculating the same very time-consuming processes over and over, SPIP stores the results of these kinds of processes in the local/cache-gd2 and local/cache-vignettes directories.
These cached images will only be deleted when the image cache is manually emptied from SPIP’s administration module, or when the parameter var_mode=images is included in the URL.