You can put here the .png files of your module:
If the picto of your module is an image (property $picto has been set to 'rollerstorage.png@rollerstorage', you can put into this directory a .png file called object_rollerstorage.png (16x16 or 32x32 pixels)
If the picto of an object is an image (property $picto of the object.class.php has been set to 'myobject.png@rollerstorage', then you can put into this directory a .png file called object_myobject.png (16x16 or 32x32 pixels)