@Roerbak I am not interested in being involved with your skin. Not because I don't like the idea, but because I don't want to interfere with what you want to create. I'd want you to have this for yourself
So instead I'll take the "teach a man to fish" approach and explain the files as good as I can:
ArrowL/ArrowR: a 12x12px image that will be used on the main menu to select the icon.
deskicon: this contains all the icons for the main menu. Each icon is 24x24 pixels and they are stacked verticaly, but only the icons at position 1, 2, 5 and 8 are used. All other icons can remain blank as the firmware will not use these anyways.
desktop: The background Image for the desktop. Icons and Text descritopns will be overlayed. From the looks of it the icons are placed 13 pixels from the left edge of the screen. The image needs to be 240x160 pixels wide.
ezoption: This is the image that will be used as a background for the settings menu. Also 240x160px
ezpdalogo: This is the splashscreen that will be shown when starting the system. Again, 240x160px
filemng; This is the background for the file selection screen. Width is 240px and the height is 160px minus the size of the header (see next line)
filemnghead: This is the header for the file selection screen. It needs to be 240px wide and the combined height of filemnghead and filemng should be 160px
icons: these contain the icons for the file manager. They are stacked 16x16px images. Most icons are obvious (First icon is the folder icon, second is for games, fourth for music files etc.) others I am not entirely certain what they are used for. My guess that in practical use, most icons are not needed anymore, though.
itemdisc: This is the image used on the main menu once you select an option. It'll show the items descrition in there. I am not 100% sure if the size can be different, by default it's 128x80px.
ok: not sure what this is used for.
Textscroll*: This is used for scroll indicators in the text viewer and the "about" screen. Since the text viewer feature is disabled, however, the about screen is the only place these are used.
textbg: The background for the text viewer/"about" screen. 240x160px in size.
The biggest issue you might run into with the skin you want to create is the text and highlight colors. Unfortunately I do not know how these can be altered