This is a common meme of all of the major Common Lisps. Emacs is a bit of an exception here, looking more like the perl/python/java model of having a bunch of discrete compiled modules that are registered with the system and loaded as necessary.
But the image nature of both CL and Smalltalk is very important, and quite different from the "rebuild it all from scratch" methodology of most programmers. With the image, you essentially have not only your program, but the entire system loaded and available to tweak and change.
For example, unlike, say, Java, in Smalltalk if you want to add a new method to a system class (say, oh, Object or String), you can easily just browse over and add a method. Voila, instantly every object or string in the system has that new message. No compiling or building or relinking or reloading. It just Is.
Obviously, this can lead to trouble if you do something silly -- with great power come great responsibility, but Smalltalks change management mechanism helps mitigate such disasters.
One could also argue that many Forth systems are image systems, they just tend to be much much smaller than ST or CL.