They want you to locate iso images in a directory on a shared VMFS volume. This way you do not have to copy the iso files to the local storage of all of your ESX servers. This does not apply if you are not using shared storage and thus you can store them on a local VMFS volume. If you with to use the vmimages directory VMware has left the legacy vmimages folder in the installation. You can override the default location of the directory (vmimages) by just selecting manual on the partitioning screen and the creating the vmimages directory on whatever local drive/partition you wish.