To clarify the terminology used for ROM:
ROM images are also used when developing for embedded computers. Software which is being developed for embedded computers is often written to ROM files for testing on a standard computer before it is written to a ROM chip for use in the embedded system
source:
http://en.wikipedia.org/wiki/ROM_image
As for Manilla, it is just an overlay UI for Windows Mobile made by HTC..aka HTC wanted to become more "user friendly" to compete with iPhone so they and other manufacturers made their own custom UIs.