For more information on your device, select it on the Support main page.
This folder is a part of the photo gallery feature and automatically created by the phone. It is used to provide fast access to images. The files in this folder are previews of the images stored on the phone. If the folder grows too big it can easily be deleted by the user by following these steps:
Select Menu > Gallery > Memory Card > mark img cache folder > Options > delete folder.
If one deletes this folder and uses the camera or gallery the folder will be created again.