In Preferences/Workbench, find the setting "Store images in..." and set it to "Other memory".
You can also save chip RAM by using a lower resolution and color depth on the Workbench screen and not loading unnecessary monitor drivers (ie, move Super72, Multiscan, Euro36/Euro72 back into Storage/Monitors instead of Devs/Monitors).
EDIT: A simpler/no WBPattern backdrop will help, too.
