Please change this value only if you understand what it means. Be aware that using this option can have a dramatic impact on the launcher's behavior, both in terms of RAM and CPU. Use with care.
Images consume a lot of memory, and it is possible to use a lot of images in Lightning (icons, backgrounds, wallpaper, etc.). In addition large images consume significantly more memory than small one. In some situation the amount of memory needed for images can exceed the app limit.
In order to help reduce the global amount o RAM needed, Lightning uses an image pool: a single image may be used in several places. For instance when switching from desktop A to B, memory for icons that become hidden in desktop A may be re-used for icons in desktop B.
While this mechanism improves global memory use, it also put more pressure on the CPU. So this is a tradeoff: the more memory available for images means the faster the app will be, the less memory in the pool means a slower app but also less memory hungry.
Please note: Using a small pool don't necessarily mean the app will use less memory: if a lot of images have to be displayed at once, then all images will have to be loaded at once too. This option will provide good results when switching between different views, such as opening a folder, close it and opening another one, or switch between desktops or the app drawer. This option may also help reduce out of memory errors (and resulting crashes) when a lot of memory is used by widgets. The overall result depends a lot on how the app is used, you mileage may vary.