I think I can save you some time here
The LCD plugin works with an on, an off and a mask image (as you already figured out I guess).
The actual definitions are made within the *.ini file (name defined in homescreen xml).
It defines which colored parts of the mask file to display when a specific time occurs. 0-23 under [HORLOGE_PATTERN] are the hours, 26-85 are the minutes.
The definitions are made using the HEX code of the colors (see mask).
If you want to have 12-hour-format, you simply copy the HEX codes of 0-11 into the 12-23 definitions. The 26-85 patterns stay the same, as the minutes are the same in both 24- and 12-hour-format.