Okay, you are working with CMP or CM2, that's great.
1) Browse with CM until you have the loco (or lco, or loc, or loo) in your list on the right side.
2) RMB click and choose "clone". This makes a copy of the loco and places your kuid in it.
3) Find the clone in the list and RMB click and choose "Edit in Explorer", a new window will open. You may have to go up one level to get to the folder containing everything, copy that folder and paste to your desktop.
4) Delete the clone from CM (if you are confident), or leave it be as a backup.
5) Open your graphics program and choose open file, then browse to the folder on your desktop, and find the graphics files there (BMP, TGA, etc...)
6)Make your changes, save.
7)If you are bringing it back to CM, you must give it a kuid # at least one digit higher than the original clone. If your clone has kuid# XXXX:1234, then you need to make this new one XXXX:1235.
8)Use CM (file) and choose import content from the menu. Import your new folder and them (edit) commit.
9)Go look at it in game or in Railyard.
10) Rinse...repeat as needed until you are happy.