Is it possible to save all rendered images (in the render buffer slots) in one operation? I have a different render in all eight render buffer slots. In the UV/Image Editor there is an option to “Save All Images” but when I select it, nothing happens. How do I configure the save path and save all the images that are in all the slots in one operation? If "Save All Images" is not for the render buffer slots then what is it for and is it possible to do what I want to do? I am trying to discover if there is something quicker than selecting each slot and saving out the individual image.
save all images  is for modified texture images, images that you have loaded from storage or packed in the .blend file and have been edited ,if no straight option is available  you can use a script to automate that process .
this script will save all non empty slots to the file path specified in the render tab and with the name displayed in the Image-editor and the number of the slot :  "Render_Image_slot_1.png"

import bpy

scene = bpy.context.scene
path = scene.render.filepath
ext = scene.render.file_extension
for img in bpy.data.images :
    i = 0
    if img.type == 'RENDER_RESULT' :
        print(img.name)
        for i in range(8):
            img.render_slots.active_index = i
            try :
                img.save_render(path+img.name+"_slot_%d"%i+ext, scene)
                print("slot %d saved"%i)
            except :
                print("Slot %d is empty"%i)


in case you need this more then once here is the Addon file for download install it and you'll find the entry in UV/Image-editor➞Image➞Save_all_slots, you can assign a shortcut for it.



to install :