Directory for module image files
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 'mymodule.png@mymodule', you can put into this directory a .png file called object_mymodule.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@mymodule', then you can put into this directory a .png file called object_myobject.png (16x16 or 32x32 pixels)