Hi some of you have proberly worked it out by now, with repainting in FS X
but to contribute with my experiance on repainting.
please read on.

maybe you guys can use my experience so far with FS X paintjobs.
To let you know how I repaint my liveries for FS X read this.

I found that if you take ex. the default A321 clean (white texture) from the texture folder, named A321_1_T.dds then you can edit that in ex. photoshop.

But first you have to take a copy of one of the ex A321 texture folders and rename it ex. texture.Delta
Then copy the clean white A321_1_T.dds file from the texture folder and paste it and replace it into your
new (texture.Delta) folder- this file A321_1_T.dds in this folder can / wil be your workfile. Move it maybe out on your desktop while working on it.

To work on that file in ex photoshop, you will as proberly mentioned by others - convert it to a graphic editing program like into an ex. psd file for photoshop.

For that, I found a useful and working program called dds converter 2 (search for it on the web)
With this program I was able to convert the dds file into an PSD file and back into the dds file.
And yes - worked fine.

When your repaint is done satisfactory for you in. You will in have to edit 
the aircraft.cfg text file to make your livery version appear in FS X as a choice in the aircraft choice window.

To do so I found in the FS X FAQ that microsoft has a program called: Microsoft Visual Basic 2005 Express Edition which I found that worked to save the edited aircraft.cfg file in.

an edited cfg file could look like this though as your next livery number in line ex:

[fltsim.6]
title=Airbus A321 Delta
sim=Airbus_A321
model=
panel=
sound=
texture=Delta
kb_checklists=AirbusA321_check
kb_reference=AirbusA321_ref
atc_id=F-DL-22
atc_airline=Delta Airlines
atc_flight_number=
ui_manufacturer="Airbus"
ui_type="A321"
ui_variation="Delta Airlines"
ui_typerole="Commercial Airliner"
ui_createdby="Microsoft Corporation"
description="The Airbus A321 is a stretched version of Airbus’ best-selling short- and medium-range A320. The A321 has a slightly increased wing area, stronger landing gear with larger tires, and engines with increased thrust. When the A321 entered service in 1988, it pioneered the use of “glass cockpit” instrumentation, the first digital fly-by-wire control system used in an airliner, and sidestick controllers

-----------

After this, your first tryout with your new livery should pop-up as an new aircraft choice. 
but change the choice of aircraft image in the ex. texture.Delta To edit that picture to be your new repaint. Take a few snapshots in air by pushing the key (v). and paste the new screendump into your ex. texture.Delta folder. It already has a picture. and thats called thumbnail.jpg. 256widex128 pxl high

the image position of the screendumps on your harddisc of your snapshots should be in
c:documents/pictures/Flight Simulator X Files

Resize, crop it and make it fit into the 256x128 format. save as .jpg

Hope you found my experience worth while reading. 
Se my own liverywork on this link and feel free to download and have fun with my FS X work. http://www.softwares.dk/fs/fs%20x%20downloads.html

P-graf