Gxx = Group Id
Ixx = Item Id
Here's a short manual on menu editing:
http://www.subsim.com/radioroom/showthread.php?t=202496
Latemail made a really big manual, but it is in German. I may have the file somewhere on my computer, but I haven't seen it for a while.
If you start editing the menu file, be sure to learn how to use this handy tool by h.sie that automatically renumbers the menu_1024_768.ini file:
http://www.subsim.com/radioroom/showthread.php?t=152823
What I would do, if I had a bigger resolution monitor:
1. Use JSGME to put Charts Addon on top of Ahnenerbe
2. See which pictures are out of place - the pictures that are located to the left side of the screen are probably ok without any changes needed
3. Take Charts Addon off with JSGME
4. Have a look at the Charts Addon folder with the TGA's to find the names of the TGA's that are in the wrong places
5. Open menu_1024_768.ini file
6. Find (shortcut Ctrl+F) the misplaced TGA's one by one
7. Edit the POS line of every misplaced TGA by adding 560 to every x value
(If it was POS=0,0,785,686 you would change it to POS=560,0,785,686)
BUT:
Leitender's Anschütz compass is a bit trickier to install, as one needs to find the correct menu groups (not the TGA's) that need changed POS values.
Additionally some of the ItemId=xxxxxxxxx values are probably duplicate with Ahnenerbe GUI, and you would need to find a new and unused hexadecimal number for those items that cause menu id conflicts.
SO:
I would recommend you make a couple of TGA's by yourself. Add just them (one or two TGA's) to Ahnenerbe, and when you learn the tricks of menu editing, then get back to Charts Addon and add the whole of the TGA's that you would like to add.